With due respect, I don't think that the interpretations you offer are correct. Matthew 5:20 and 12:36-37 are both calls to perfection. It seems that you are trying to put the passages into the context of some of the teachings in Romans regarding works of the law (since Jesus in both passages is dealing with Pharisees), but I don't think they really fit here.
A hidden premise in your argument, I think, is that man cannot possibly be expected to be perfect in anything. But this premise is itself not really supported in Scripture. I lay out my reasoning below.
Interpretation of Matthew 5:20
For I say unto you, That except your righteousness shall exceed the righteousness of the scribes and Pharisees, ye shall in no case enter into the kingdom of heaven.
We understand from this verse that Jesus is not teaching that in order to get into heaven you must be more holy and do more works that the scribes and Pharisees, rather it is intended to put righteousness by works out of the reach of every human.
I think you are reading a lot into Matthew 5:20. If you include the previous verses, it can be construed to mean exactly the opposite of what you suggest:
For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled. Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven
Regarding "works", in the same passage Jesus says:
Whosoever therefore shall break one of these least commandments, and shall teach men so, he shall be called the least in the kingdom of heaven: but whosoever shall do and teach them, the same shall be called great in the kingdom of heaven.
Elsewhere Jesus makes it clear that man will be judged according to his works. Matthew 25:36-41 is one example. Another is Matthew 16:27:
For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man according to his works.
This same teaching echoes the Old Testament (e.g. Psalm 62:12, Job 34:11) and is echoed by the Apostles (e.g. Romans 2:6, Revelation 20:12, Revelation 22:12).
The point of Matthew 5:20 is not, I think as you suggest, "to put righteousness by works out of the reach of every human", but rather to prescribe "works" - if we can call commandments "works" - that are still more demanding than those found in the law. This is clear from the verses that follow:
Ye have heard that it was said by them of old time, Thou shalt not kill; and whosoever shall kill shall be in danger of the judgment: But I say unto you, That whosoever is angry with his brother without a cause shall be in danger of the judgment ... (Matthew 5:21-22)
Ye have heard that it was said by them of old time, Thou shalt not commit adultery: But I say unto you, That whosoever looketh on a woman to lust after her hath committed adultery with her already in his heart. (v.27-28)
It hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement: But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery. (v.31-32)
Again, ye have heard that it hath been said ... But I say unto you ...(v.33-34)
Ye have heard that it hat been said ... But I say unto you ... (v.38-39)
Ye have heard that it hath been said ... But I say unto you ... (v.43-44).
Interpretation of Matthew 12:36-37
But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment. For by thy words thou shalt be justified, and by thy words thou shalt be condemned.
... verses 36 and 37 seem to indicate the same premise of “put good works out of reach”.
If I understand you correctly here (which I may not), you are saying that since it is unreasonable to expect that one not speak a single unrighteous word, Jesus must be teaching that it is unreasonable to think that good works are within our reach - in other words, that man cannot be expected to be perfect.
But this directly contradicts another teaching of Jesus recorded by Matthew: Be ye therefore perfect, even as your Father which is in heaven is perfect. (Matthew 5:48).
Furthermore, what Jesus is teaching here is nothing new. It was recorded in Job and echoed in Ecclesiastes:
Thine own mouth condemneth thee, and not I: Yea, thine own lips testify against thee.
The words of a wise man’s mouth are gracious; but the lips of a fool will swallow up himself.
One Greek commentary on this passage reads:
Here He strikes fear into our hearts, that we will give an account for even a careless word, that is, any lying, slanderous, indecent, or mocking word. Then He brings forth testimony from Scripture, lest He appear to be speaking His own words: "By thy words thou shalt be deemed righteous, and by thy words shalt thou be condemned."1
The connection with the "unpardonable sin" is that if simple men (Gr. ἄνθρωποι) will be held accountable for each idle word at judgment, all the more so the Pharisees. Jerome comments:
This, too, goes with what was said before, and the meaning is that on the day of judgment each person must render an account of his or her words. If an idle word which by no means edifies the listeners is not without harm to the speaker, how much more will you Pharisees, who criticize the works of the Holy Spirit and say that I cast out devils by Beelzebub the prince of devils, have to render an account of your criticism? An idle word is what is spoken without benefit to the speaker and the listener. We overlook serious things and utter frivolous things and tell old wives’ tales. One who acts like a buffoon and makes mouths drop with boisterous laughter and who utters disgraceful things—that person shall be held to account, not for an idle word but a slanderous word.2
You will find a similar understanding of this verse in the writings of virtually all of the early Church Fathers, including Irenaeus (130-202)3, Cyprian of Carthage (200-258)4, and John Chrysostom (c 349-407)5.
1. Theophylact, Explanation of the Holy Gospel According to St. Matthew (tr. from the Greek; Chrysostom Press, 1992), p.107
2. Commentary on Matthew II.XII.36
3. Against Heresies II.XIX.2, IV.XVI.5
4. To Quirinius III.13
5. Homilies on Matthew XLII, LXXXVI