This issue is addressed in the Augsburg Confession, which the LCMC accepts as "normative for our teaching and practice," like many other Lutheran groups. First we'll briefly look at the "evil human imaginations" part of this, before looking more closely at why unsaved people can accomplish good things, which seems to be the main point of your question.

Evil imaginations

You've actually provided plenty of evidence for "why the human imagination is considered evil" by Lutherans; they cite similar passages as well as the writings of the church fathers to defend the statement found in AC II:

since the fall of Adam all men begotten in the natural way are born with sin, that is, without the fear of God, without trust in God, and with 2] concupiscence; and that this disease, or vice of origin, is truly sin, even now condemning and bringing eternal death upon those not born again through Baptism and the Holy Ghost.

"Concupiscence" is not a word we normally use, but the Defense of the Confession explains that it is that "which seeks carnal things contrary to God's Word" – that is, the natural desires of man tend naturally toward sin. These things are "abiding defects"; our hearts are "naturally destitute of love, fear, and confidence in God." The full defense of article II cites 1 Corinthians 2:14, Romans 7:5, and other passages, as well as several church fathers.

Good things done by the unsaved

So how, then, is anything "good" done by unbelievers? This issue is addressed in Article 18. It opens by accepting that even sinful man can do "civil righteousness," but not "spiritual righteousness":

Of Free Will [our churches] teach that man's will has some liberty to choose civil righteousness, and to work 2] things subject to reason. But it has no power, without the Holy Ghost, to work the righteousness of God, that is, spiritual righteousness;

This is because the "natural man," or man in his unsaved state, cannot be spiritually righteous without the Spirit of God:

since the natural man 3] receiveth not the things of the Spirit of God, 1 Cor. 2:14; but this righteousness is wrought in the heart when the Holy Ghost is received 4] through the Word.

The Confession appeals to Augustine on this point, and quotes him as arguing that it is "reason" that allows men to perform many "good" "works of this life," and that category includes many positive things that are accomplished by the unsaved:

We grant that all men have a free will, free, inasmuch as it has the judgment of reason; not that it is thereby capable, without God, either to begin, or, at least, to complete aught in things pertaining to God, but only in works of this life, whether good 5] or evil. "Good" I call those works which spring from the good in nature, such as, willing to labor in the field, to eat and drink, to have a friend, to clothe oneself, to build a house, to marry a wife, to raise cattle, to learn diverse useful arts, or whatsoever good 6]pertains to this life.

We might ask, what is the source of these "good" things? Augustine (and Augsburg) continue by saying that it is the providence of God:

For all of these things are not without dependence on the providence of God; yea, of Him and through Him they are and have their being.

This is an expression of the doctrine of common grace, which teaches that God bestows many blessings (like nature and human reason) widely on mankind, not just on those that are saved. But nature and reason have limits – because the unsaved "are born in sin" (AC II), these blessings can produce the "outward work," but

cannot produce the inward motions, such as the fear of God, trust in God, chastity, patience, etc.