How could a loving, just, merciful God have created such an unfair test for humans?

Stick with scripture. John 3. You must be born again. Even if you think you are a good baptist or catholic, it's pointless. Jesus only. Not anything to do with your works. Ephesians 2:8-9. Don't make this too hard by trying to sound philosophical driven by intellectual or moral blindness.

I think that the contradictions you perceive in God's nature is the result of a misconception about God's justness in relation to our sinfulness.

The Bible teaches that:

No matter how "good" we think we are, no one is righteous, and all have sinned.

Romans 3:10-12 (NIV)
¹⁰ As it is written:
   “There is no one righteous, not even one;
¹¹ there is no one who understands;
   there is no one who seeks God.
¹² All have turned away,
   they have together become worthless;
there is no one who does good,
   not even one.”

Romans 3:23 (NIV)
²³ for all have sinned and fall short of the glory of God,

Because of our sin, we all deserve death

Romans 6:23 (NIV)
²³ For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord.

It is only by immense grace that God sacrificed His own Son to die for our sins. This sacrifice was necessary in order to satisfy His justness. For if he simply ignored our sin, he would be merciful, but he would not be just. So God is just because He requires payment to be made for sin, and He is loving and merciful because He sent His own Son to make that payment on our behalf.

My point here is that the fact that some people are less likely to come to Christ is not injustice on God's part - it's justice. Even if God only bestowed mercy on a handful of individuals in all of history, it would still be a display of infinite mercy, far beyond what we actually deserve.

Let me try to illustrate this with an analogy. Say there's a judge who presides over ten criminal cases. For nine of them, he assigns them their due punishments. But for one of them, he has extraordinary compassion, and undergoes the man's punishment for him. Should we say that the judge was too harsh on the other nine? No, he judged them rightly. The other nine might think it unfair that grace was given to the one (Matthew 20:1-16), but isn't it better that grace be given to one that to none?

See also this answer.

In the Orthodox tradition, it is understood that in the Resurrection, Christ descended into Hades, the realm of the dead, where he preached the Gospel to the dead of all times. Those who die in Christ have in their baptism already participated in his passage through Hades, which is death, and thus do not taste of death as Christ promises in the Gospels.

Given that Hades is not merely some temporal place, but rather the place which is the state of death, we acknowledge that all who died outside of Christ and will die outside of Christ have and will have had the Gospel preached to them by Christ when he descended into Hades which in our time happened around 2000 years ago.

These, even those who lived after, may have been some of the righteous souls seen risen from their graves as accounted in Matthew.

Thus we see that 'there is no path to the Father except through Jesus' but essentially all persons are given the chance. A person who was a good person in their life and not merely self-righteous is the sort we hope would accept the Gospel when the Lord himself delivers it.

But we can only hope in the goodness of God and we cannot dogmatically declare the state of such souls.

We can, however, and should, pray for them.

--- I was asked for some evidence of this position.

First, some Orthodox evidence of this position (as being a legitimate teaching of the Orthodox Church). The Troparia of Pascha simply says, 'Christ is risen from the dead, trampling down death by death, and upon those in the tombs bestowing life.'

Here is a link to the Paschal hours which is pretty explicit about what happened:

http://en.wikipedia.org/wiki/Paschal_Hours

In the tomb with the body and in Hades with the soul, in Paradise with the thief and on the throne with the Father and the Spirit, wast Thou, O boundless Christ filling all things.

Two, here are the scriptures:

http://cantuar.blogspot.com/2012/04/8-bible-verses-on-christs-descent-into.html

He has done a good job summarizing it.

In particular:

Ephesians 4:9

Now that he ascended, what is it but that he also descended first into the lower parts of the earth?

And Peter in Acts 2:24

Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.

And Peter again in his Epistle (1 Peter 3:19,20) :

By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.

It is death is referring to Hades, not merely being dead, and that those preached to were those previously dead.

Regarding the somewhat more controversial point of the timeless nature of the descent, read

http://orthodoxeurope.org/page/11/1/5.aspx

In particular see this:

Has this anything to do with those who died outside Christian faith after the descent of Christ into Hades? No, if we accept the Western teaching that the descent into Hades was a ‘one-time’ event and that the recollection of Christ did not survive in hell. Yes, if we proceed from the assumption that after Christ hell was no longer like the Old Testament sheol, but it became a place of the divine presence. In addition, as Archpriest Serge Bulgakov writes, ‘all events in the life of Christ, which happen in time, have timeless, abiding significance. Therefore,

the so-called ‘preaching in hell’, which is the faith of the Church, is a revelation of Christ to those who in their earthly life could not see or know Christ. There are no grounds for limiting this event… to the Old Testament saints alone, as Catholic theology does. Rather, the power of this preaching should be extended to all time for those who during their life on earth did not and could not know Christ but meet Him in the afterlife[Bulgakov].

According to the teaching of the Orthodox Church, all the dead, whether believers or non-believers, appear before God. Therefore, even for those who did not believe during their lifetime, there is hope that they will recognize God as their Saviour and Redeemer if their previous life on earth led them to this recognition.

This is essentially affirmed in the Paschal hours where we say:

Bearing life and more fruitful than Paradise, brighter than any royal chamber, Thy tomb, O Christ, is the fountain of our resurrection.

I believe the best way to address your question is to take apart what you've written, and address it piece by piece, looking especially at the assumptions you have made going into your argument. Please understand that while you may not agree with many of the statements I will make here, they do represent a Christian perspective. In particular, I will be writing from a Reformed Protestant perspective, and using some of their language. Most of the major concepts can be found in many other Christian traditions as well, although the presentation and emphasis varies. Also, you will not find unanimous agreement on these points, as many conflicting ideologies make a claim to be Christian. At some point, you must dig into the evidence and decide who's holding the picture right-side-up. I don't claim to have everything right, but I do claim that the following major points both answer your question and represent the way the world really is -- that when you look at the world this way, you suddenly find the view is in focus.

Let's start with your first point:

"God is love"

This is true. However, the definition and nature of love is often misunderstood in secular culture, and the theological claim that God is love must be understood inside the context of what else we understand about the nature of God. Most importantly, in order to love, one must also hate. Love is a defining feature of God, but in order to love a good thing, one must hate whatever is opposed to that thing. You cannot love Jews without hating the Holocaust. You cannot love babies and not hate when they are murdered. You cannot love good without hating evil. You cannot love righteousness without hating sin.

This brings up an issue you skipped over in your second point. 2 Thessalonians 1:6 deserves a little attention before we move on to the merciful bit you emphasized:

"God is just: He will pay back trouble to those who trouble you"

The justice of God is a big topic, but it strikes at the heart of your question. You ask, "How could a just God do something unfair?". Christianity's answer is that he did not. What God does is entirely fair, and he even explains why. Even the second part of this verse has a clue about what's going on. He groups men into more than one category. The verse is NOT addressed to all of humanity. It is addressed to a group of believers -- in this case, a specific gathering of believers (or church). With this in mind, you can see -- even in the fragments of text that you quoted -- that God treats some people one way, and some people another.

Your question goes on to emphasize the ways God shows compassion. For example, you used Deuteronomy 4:31:

"[God] will not abandon or destroy you or forget the covenant with your ancestors..."

Even here, I hope you see a hint of the answer. That covenant he references wasn't something God made with humankind, it was something he made with a subset of the human race. Originally, this promise was focused around one line of family descendants (Abraham to Isaac to Jacob etc.). Through history, you can trace this promise that is now open to a much broader swath of humanity -- namely, all of those that, through Jesus, have been adopted into that same family.

So let's move on to your issue with John 14:6:

"being a good person [...] is not enough"

This highlights the biggest piece of the puzzle that you are missing. There are no good people. None. Anywhere. From Adam, right on down the line to you and me, all have sinned and missed the mark, fallen short of the standard of what a good person must be in order to qualify to be accepted by God. All, that is, except one. Jesus Christ. The thing that is most emphatically different about Him is that He was without sin.

Let's stop there, and go back to 'fairness'. One of the core tenets of the Christian faith is the unapologetic claim that ALL MEN, having trespassed against God, justly deserve His wrath. In fact, it would be unjust of God to not punish all men. In this sense, to be merciful would be a disgrace to justice. So how can God also be merciful? For that, we go back to Jesus.

Jesus satisfied the divine demands of living a 100% sinless life, then He surrendered Himself, and was made to be sin in our place. Having rightly deserved everything we do not, He took on Himself the iniquities of men, and was punished for them. The wrath of God was emptied upon him. Forget the cross and the Romans and the whips and the crown of thorns for a minute, because those were only props -- hints and clues at what the real story is about. He drank from the cup of God's wrath, so that men wouldn't have to.

So now we come to the issue of who. All of humanity stood condemned. Jesus came and said "See that man over there, and that one too, and those over there ... I want them to be mine -- and I'm willing to pay." God granted Him His wish, allowed Him to pay the price (even though it meant subjecting His own beloved Son to torture and death, both at the lowly hands of created men, and at His own hand, as He crushed Him with the punishment for sin).

So if Jesus says "I paid for that one.", who are we to argue and say it isn't fair that He doesn't choose another? We do know several things about His choice:

• He doesn't base His choice on the good works or worthiness of individual people.
• He does not leave it up to men to find Him, but goes out and hunts down His lost sheep, however far out of the fold they might be.
• He does often extend His mercy and call through to descendants of ones He has already called and saved, but this is not a guarantee. Some that grew from the same branches are also pruned.
• It isn't the job of Christians to do the picking and choosing. In fact, quite the opposite: those who have been chosen are also commanded to go out and proclaim what Christ did to ALL: announcing to them that there is One who has taken away the sins of the world, and that through Him they might be saved.

There are quite a number of other points that could be made about your question. I'll address a few more in brief:

"those who are simply unable to believe in any god"

The Bible does not acknowledge the validity of atheism. On the contrary, it states that all men, whether they acknowledge it or not, have an innate knowledge of God. It also says that all of creation declares Him, albeit in a general way. Quite simply, this is not a valid excuse.

"Belief isn't a choice..."

After a fashion, I would actually agree with this, but not in the way you are thinking. I would say men did have a choice, and they choose wrong. And now you don't have the power to make a right choice. In that sense, without God's intervention, you cannot just up and decide to change your fate. One theologian humorously addresses this point this way:

Augustus Toplady: “A man’s free will cannot cure him even of the toothache, or a sore finger; and yet he madly thinks it is in its power to cure his soul.”

Adam was a prototype; you are of the same stock. You choose, and continue to choose, sin. Your own free will, left to its own ends, will never choose anything else. Yet Christianity calls men to make a new choice, to believe in Christ, to put their trust in Him, and stake their lives on the work that He did in removing our guilt from sin in the eyes of God. It also holds that this is only possible through a supernatural work --the intervention of the Holy Spirit-- in which God rips out your faithless heart of stone, and gives you a heart of flesh that is able to believe, love and obey -- but this, being His work, does not excuse you from searching out, wanting, and submitting to this new birth.

"the fair way would be for him to appear to every human at birth"

Who are you to say what is or isn't fair? What is a pot to say to the potter that "you should have made me with a handle"? There are some aspects of faith that are a mystery --that we don't have answers for-- but even human logic could present an argument about this scenario being 'fair'. If the outcome you expect is for everybody to be united under one unanimous belief, then whatever God must have done to make that happen would have destroyed any pretense of free-will they had. Would you have us all be robots? Is being a free-willed but fallen human who is then chosen to be resurrected from a living death and eventually restored to perfection in Glory not enough drama and beauty for you? Will you not bow in recognition that the Christ who did this is Lord -- and do so now willingly... before being left no other choice?

TL;DR: All of God's creation stands in rebellion of Him, and justly deserves to be destroyed. Only one Man was ever sinless, and that Man laid down His life so that others might live. How He picks who to adopt isn't the business of the newly adopted children. It is their job to turn around and tell others that He's still in the adoption business. If "fairness" is being stretched, it is being stretched in the direction of no-one deserves anything, so anything we get is all grace.