How do proponents of the “free-will defense” against the problem of evil explain that God can be free and immune to moral evil at the same time?

Revelation 22:13 I am the Alpha and the Omega, the first and the last, the beginning and the end.

God's will is perfect - His will is done (Isaiah 55:8-9)(Proverbs 19:21)(Jeremiah 29:11)(Proverbs 16:4)

Numbers 23:19 God is not human, so He does not lie. He is not a human being, so He does not change His mind. Has He said,and will He not do it? Or has He ever promised and not carried it through?

Your argument is based on two misconceptions, first, God is incapable of committing evil, and second is the mis-definition of omnibenevolence.

First, may be resulted from a popular misconception among Christians themselves that God is logically incapable of doing evil or sinful works, such as acting as Satan, his arch-enemy. However, we don't see any good reasons for this. Anywhere the scripture describes his perfect infallible character, it is not due to a deductive inference of his handicap and disability to commit wrongdoings/mistakes, but rather for his holy and faithful character which is truly worthy of worship from our inductive experience. If God turns out to be incapable of failing, then there he hardly worthy of any praise. The same goes for the incarnate Son, Jesus who perfectly reveals the character of God. God should be understood as holy and perfect due to his wilful holy character. His moral character shouldn't be limited and locked in our logical paradigms of necessity, especially when it takes away his freewill.

In Matthew 4, the story of Jesus' temptation by Satan, we see his refusal to accept Satan's offer to rule the earthly world by turning evil. He does so, not due to his handicap or infallibility, but due to his perfect will and obedience like every other righteous man has done in history.

It is a common misconception among Christians who believe Jesus was incapable of sinning despite being a true human. (You, see your argument can be better formulated to find contradiction between Jesus' human nature with freewill or even with the quality of deserving praise and honour). The traditional or popular Christians such as Plantinga and William Lane Craig would rationalize their contradictory views by resorting to the Gnostic roots of their religion which reduces the humanity of Christ. He was not truly human as the Bible describes, Hebrews 2:14-18, 4:14-15 but was having a reduced superficial human nature, also see the unscriptural special pleading of original sin exemption for Jesus under the same Gnostic world-view:

Heb 2:14-18 ESV

¹⁴Since therefore the children share in flesh and blood, he himself likewise partook of the same things, that through death he might destroy the one who has the power of death, that is, the devil, ¹⁵and deliver all those who through fear of death were subject to lifelong slavery. ¹⁶For surely it is not angels that he helps, but he helps the offspring of Abraham. ¹⁷Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people. ¹⁸For because he himself has suffered when tempted, he is able to help those who are being tempted.

[Hebrews 4:14-15 ESV] Since then we have a great high priest who has passed through the heavens, Jesus, the Son of God, let us hold fast our confession. For we do not have a high priest who is unable to sympathize with our weaknesses, but one who in every respect has been tempted as we are, yet without sin.

The word omnibenevolent simply means all-loving, or the greatest in loving kindness. It doesn't mean incapable of doing wrong or evil. The traditional conception of Satan too is perceived as a rebellious angel of God, where angels are also associated with similar heavenly or divine like nature of Elohim as companions to God. I think associating a logical infallible quality to God takes away from his quality of perfection and worthiness, as a robot lacking freewill is hardly worthy of any praise. Thus, you seem to be reasonable in your objection.

All-loving, or infinitely good, usually in reference to a deity or supernatural being, for example, 'God'. Its use is often with regards to the divine triad, whereby a deity is described to be simultaneously omniscient, omnipotent and omnibenevolent. This triad is used especially with the Christian god, Yahweh. The Century Dictionary.

Isa 45:7-9 RV

⁷I form the light, and create darkness; I make peace, and create evil; I am the LORD, that doeth all these things. ⁸Drop down, ye heavens, from above, and let the skies pour down righteousness: let the earth open, that they may bring forth salvation, and let her cause righteousness to spring up together; I the LORD have created it. ⁹Woe unto him that striveth with his Maker! a potsherd among the potsherds of the earth! shall the clay say to him that fashioneth it, What makest thou? or thy work, He hath no hands?

The word H7451 רַע ra` (rah) for evil has been translated "calamity" in the new Bible versions, whenever it applies to God (for apologetical reasons), for it doesn't mean evil in the same sense as we tend to perceive it to be, as wickedness, sinful, satanic. It only means calamities and destructions. God is not wicked, immoral, imperfect, even if he destroys mankind, as he will do eventually, for God he must have justifiable reasons for it.

When we can trust the wicked governments in their conspiracies to oppress us, we can surely trust God with all our faith and love, even if he destroys us, as this mortal life isn't the ultimate possession we have. This is the teaching of the whole scripture, starting from the most fundamental book of Job.

The logical argument

The OP presents a logically valid argument to show a contradiction between God having free will and God being omnibenevolent. However, the argument is not sound because premise 2 is false.

Premise 2 (God is omnibenevolent => God is not capable of moral evil) is an untenable definition of omnibenevolent--it in fact begs the very question the argument is trying to evaluate. If God expressed love, provided aid, and sustained life simply because He couldn't not do those things, He would be no more benevolent than oxygen, which also sustains life...but is not benevolent.

Oxford languages defines benevolent as "well-meaning and kindly". Something which sustains life because it is compelled to do so by the laws of nature may be useful, but it fails to demonstrate any evidence of benevolence. For a more down-to-earth example, we might consider how we would feel if someone did a great service for us (oh, how benevolent!), only to learn that they were coerced into doing so (it no longer appears benevolent at all). Neither oxygen nor someone forced into a service project are well-meaning & kindly. It is a category error to assume Christians use "benevolent" to describe a God who is loving, merciful etc. because He is compelled to do so.

God is benevolent because He has free will and chooses to do things that are well-meaning and kindly.

Why not create humans with perfect character

The follow up question--why human beings are not created with a perfect character from the outset?--is heavily influenced by one's specific theology. This post will answer the question from the perspective of a member of The Church of Jesus Christ of Latter-day Saints.

The Pre-mortal Existence

My denomination teaches that we existed as spirits prior to physical birth. As taught in The Family - A Proclamation to the World:

In the premortal realm, spirit sons and daughters knew and worshipped God as their Eternal Father and accepted His plan by which His children could obtain a physical body and gain earthly experience to progress toward perfection and ultimately realize their divine destiny as heirs of eternal life.

We had not then developed to the point we are at now, and we are not now everything we can become.

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Creation ex-nihilo

My denomination does not believe in creation ex-nihilo (the downvote button is above and to the left) with respect to the universe generally or humankind specifically (a brief discussion of creation ex-nihilo here, a longer discussion here).

I believe that God creates by organizing, separating, & transforming. In almost every context this is what is meant by the word "create". To create is to take something that already exists and transform it. In another post I wrote about the concept of an "eternal self" - that there is a fundamental part of us that has always existed and will never cease to exist. We are eternal beings: we grow and develop, but we do not begin or end.

Man was also in the beginning with God. Intelligence, or the light of truth, was not created or made, neither indeed can be. (Doctrine & Covenants 93:29)

The elements are eternal (Doctrine & Covenants 93:33)

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Theosis

My faith is unique in its understanding of theosis. We believe that we are the spirit children of God and that we can become like Him. We believe very literally Paul's statement "And if children, then heirs; heirs of God, and joint-heirs with Christ" (Romans 16:9, see also 1 John 3:2-3, Moroni 7:48).

Theologian and former Brigham Young University Dean of Religion Robert Millet has written:

Latter-day Saints do not believe that human beings will ever be independent of God, or that they will ever cease to be subordinate to God. They believe that to become as God means to overcome the world through the atonement of Jesus Christ (see 1 John 5:4—5; Revelation 2:7, 11). Thus the faithful become heirs of God and joint heirs with Christ and will inherit all things just as Christ inherits all things...There are no limitations on these scriptural declarations; those who become as God shall inherit all things (see here p.15)

The Church's statement on the topic can be found in the essay: Becoming Like God.

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Conclusion

God's plan is not one in which we were created out of nothing -- some essential part of who we are has always existed and God met us where we were. It is a merciful plan in which it is possible for someone to become more than they were in the past or are in the present. If the only things that could have perfect character were those which started with perfect character, we'd be talking about a much less hopeful plan and a much less powerful God.

Why didn't God create humans with perfect character? Because we already had character and it wasn't perfect. We are eternal beings. God has endowed our "eternal selves" with spiritual & physical faculties, but He has not and does not compel us to change our character. His plan provides us with the opportunity for the transformation of our character, if we are willing.

The great miracle of this plan is not that God creates something that does what He wants it to do, but that He can take someone that does not do what He wants them to do, and develop, teach, lead, and coach them to do as He does of their own volition, and that for those willing to walk that path, God has the power to make them as He is.

Post-script

This post might naturally lead to follow-up questions about how & why the nature of God is what it is. This appears to be outside the scope of the original question, but may make for a useful follow up question.

Disclaimer: these thoughts are the product of my own study and do not constitute official statements by The Church of Jesus Christ of Latter-day Saints

This is an old question which St. Thomas Aquinas deals with in the Summa Theologiae; more specifically, at I, q. 19, a. 10, ad 2 ("Whether God has free-will?):

Objection 2. Further, free-will is the faculty of the reason and will, by which good and evil are chosen. But God does not will evil, as has been said (Article 9). Therefore there is not free-will in God.

[...]

I answer that, We have free-will with respect to what we will not of necessity, nor be natural instinct. For our will to be happy does not appertain to free-will, but to natural instinct. Hence other animals, that are moved to act by natural instinct, are not said to be moved by free-will. Since then God necessarily wills His own goodness, but other things not necessarily, as shown above (Article 3), He has free will with respect to what He does not necessarily will.

[...]

Reply to Objection 2. Since the evil of sin consists in turning away from the divine goodness, by which God wills all things, as above shown (De Fide ii, 3), it is manifestly impossible for Him to will the evil of sin; yet He can make choice of one of two opposites, inasmuch as He can will a thing to be, or not to be. In the same way we ourselves, without sin, can will to sit down, and not will to sit down.

Free will only applies to what does not come out of necessity. God, by His very nature, is necessarily (and perfectly) good, and therefore God wills His own goodness not freely but out of necessity. However, there are things which God does not do out of necessity but voluntarily, such as creation, which He could have or have not done (as explained in the aforementioned Article 3). This is enough for God to have free will. Creatures can't be necessarily good, they can always fall away from perfection one way or another, therefore all creatures with free will (i.e. angels and humans) can freely fall away from good. Similarly, there also are things which rational creatures will out of necessity rather than voluntarily (cf. ST I, q. 83, a. 1, ad 5). In either case, free will isn't absolute in the sense that everything willed is free.

But, in sum, the free will of God and the free will of creatures are not exactly the same. What applies to one does not necessarily apply to the other, and this is very important to remember when analyzing God's attributes in general.

Your issue, as you might expect, is with premise 4.

A being with free will does not necessarily need to be capable of moral evil. The free will defense doesn't require this premise, and you're right in raising this objection to a version of the free will argument that attempts to use it.

As Plantinga says in The Nature of Necessity, "A world containing creatures who are significantly free (and freely perform more good than evil actions) is more valuable, all else being equal, than a world containing no free creatures at all." This is the premise required to uphold the free will argument, and take care to note that the premise doesn't apply to God Himself. This degree of freedom, which includes the freedom to choose moral evil, is presented as a valuable quality for creatures to posses, but not as something which God must posses.

A couple of footnotes about free will, omnipotence, and omnibenevolence

It's important to get these terms right, or otherwise we can start constructing arguments that don't work the way we think they do. Free will is not in itself the ability to choose anything at all. Clearly there are options which are either logically impossible or physically impossible for us to choose. Instead, we can only make choices within the bounds of our nature. The free will defense does not require or expect that all beings have the ability to choose moral evil, though in Christian thinking it's certainly the norm to believe that all created beings have free will that doesn't include an inherent omnibenevolence. (Interestingly, this suggests that there is a radical preference for created beings to be able to freely choose God, so much so that God hadn't even bothered to create beings which were themselves omnibenevolent in nature)

God's free will does not include the ability to choose evil because it is inherently incompatible with His own substance. This does not, properly understood, contradict with his omnipotence. God is no less free or less powerful by abstaining from exercising his power towards evil. Omnibenevolence is not a limit on God's power, but a direction towards which that power is aimed.

Evil is defined as 'contrary to God's will' ergo it is impossible for God to be evil and his having free will has no effect on this.

The point often at issue is in fact this definition of evil. Satanists, atheists, self-is-God Marxists, self-is-God Humanists, self-is-God New Age hippies and those of many more religions will all happily tell you that some will or act of God is evil but they are using a different (subjective) definition of evil. Usually 'anything I disagree with strongly'.

To paraphrase an idea from Dale Renlund, God does not simply want us to be well-behaved pets in the celestial living room, but to be full heirs of His glory (see Romans 8:17).

This life, then, is a preparation for that future state described in 1 John 3:

2 Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is.

3 And every man that hath this hope in him purifieth himself, even as he is pure.

God's plan offers a means for fallen people like us to be transformed into the kind of people described by John. The fallen world we live in is a refiner's fire which, if properly applied, serves to develop and refine us. The refiner's fire wasn't plan B because things went south in Eden -- it was the original plan. The future state described by John is a place we haven't reached yet.

God is already there--He is pure. He has no guile. He doesn't get tricked or succumb to temptation. That which He intends, He does. My faith teaches that the first principal of the Gospel is faith in the Lord Jesus Christ. Holy writ & personal inspiration give us reason to place our faith in Him. As already noted by Mason Wheeler, Jesus was tempted and yet did no sin. There's no incompatibility in a Being having free will, and consistently using it to do good. That's a skill God is teaching us.

God doesn't create human beings which are free and have perfect character [at the outset] for the same reason He doesn't create square circles--a spiritually immature recipient of eternal life is a contradiction in terms. Like a good father training up a son before giving him great responsibility (see Matthew 25:14-30), God is preparing us for what He wants to give us. Character is something that is being painstakingly developed in us (refiners fire) at great cost (the blood of God) so that we can inherit eternal life.

You're switching your morality definitions between your premises.

One counter to your premises that shows this comes from divine command theory which, essentially, states that God's will is the definition of moral good. This is then also the definition of good which defines the omni-benevolence of God.

This means that premise 4 doesn't necessarily follow. Yes God has free will and could choose to do many things that non-christian morality would deem evil but not by the definition of good we're using for omni-benevolence.

There is no restriction in action, only in definition. Boiling your question down you're asking "Could God wish to do a thing that is against God's will." which is a logical discrepancy. In this framework God is not good in some external sense that you or I may be considered good, God is good in a definitive sense. Goodness is God (by the definitions of divine command theory).

Hope this helps clear things up.

This question is answered directly in the text of the Bible.

14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession.

15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin.

-- Hebrews chapter 4

The "missing link" here is character. God has free will, and is fully capable of being tempted and enticed to do evil, but consistently chooses to use that free will in benevolent and righteous ways. In this, and particularly through the example of the life of his son, Jesus Christ, he teaches us how to use our free will in benevolent and righteous ways as well.