Due to limited time, I will not be presenting opposing viewpoints in this post. I will simply give my take on Mt 5:18 and cite one scholar, who seems to put forth a view that's in harmony with the NWT rendering of the Matthean text. There is plenty of material out there on this verse; some agree with the rendering found in the NWT while others do not (cf. Hagner's Word Commentary on Matthew).
Firstly, let us compare how different Bible translations render this passage:
"For assuredly, I say to you, till heaven and earth pass away, one jot or one tittle will by no means pass from the law till all is fulfilled" (NKJV).
"For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished" (NASB).
"For truly, I say to you, till heaven and earth pass away, not an iota, not a dot, will pass from the law until all is accomplished" (RSV).
"for, verily I say to you, till that the heaven and the earth may pass away, one iota or one tittle may not pass away from the law, till that all may come to pass" (YLT).
"I tell you the truth, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished" (NIV).
"Amen, I say to you, until heaven and earth pass away, not the smallest letter or the smallest part of a letter will pass from the law, until all things have taken place" (Catholic NAB).
"I assure you, heaven and earth may as well cease to be, as that one jot or one tittle of the law should fail of its completion" (Mace NT of 1729).
"Truly I say to you that sooner would heaven and earth pass away than for one smallest letter or one stroke of a letter to pass away from the Law until all things take place" (NWT 2013 Revision).
As you can see, translators often choose to translate the first occurrence of ἕως ἂν in a manner that suggests "heaven and earth" will or might eventually pass away. But the NWT treats the language of Mt 5:18 as an example of hyperbole.
The Greek of Mt 5:18 reads:
ἀμὴν γὰρ λέγω ὑμῖν, ἕως ἂν παρέλθῃ ὁ οὐρανὸς καὶ ἡ γῆ, ἰῶτα ἓν ἢ μία κεραία οὐ μὴ παρέλθῃ ἀπὸ τοῦ νόμου, ἕως ἂν πάντα γένηται.
Ulrich Luz (Matthew 1-7: A Commentary. Translated by Wilhelm C. Linss. Edinburgh: T & T Clark, 1989) reviews the translational possibilities for the Greek of Mt 5:18. He observes that the words ἕως ἂν παρέλθῃ ὁ οὐρανὸς καὶ ἡ γῆ can either be a periphrastic way of saying "never" (see Luz, p. 265ff) or it may "limit the validity of the law until the end of the world."
Luz points out that the decision between these two alternatives is "very difficult" since in Mt 24:35, Jesus is reported to have said: "Heaven and earth will pass away, but my words will not pass away." Elsewhere, however, we read:
"But it is easier for heaven and earth to pass away, than for one stroke of a letter in the law to be dropped" (Lk 16:17 NRSV).
The Hebrew Scriptures also teach us concerning the Messiah:
"In his days may the righteous flourish, And abundance of peace till the moon is no more" (Ps 72:7 NASB).
Yet we read in Jeremiah 31:35-36 (ASV):
"Thus saith Jehovah, who giveth the sun for a light by day, and the ordinances of the moon and of the stars for a light by night, who stirreth up the sea, so that the waves thereof roar; Jehovah of hosts is his name: If these ordinances depart from before me, saith Jehovah, then the seed of Israel also shall cease from being a nation before me for ever."
The passages in Matthew 24:35; Lk 16:17 and the OT usage of ἕως ἂν in the LXX might have influenced the NWT handling of Mt 5:18--however, going back to Luz' discussion, knowing exactly how to construe the language of Mt 5:18 does seem extremely difficult. As he says:
"Does the evangelist mean that--in contrast to the words of Jesus--the law is to be valid only until the passing of heaven and earth? Matthew then would follow a sparsely documented Jewish idea that the law would be abolished in the future eon" (ibid).
In order to resolve the apparent or imagined difficulties associated with translating and understanding Mt 5:18, Luz goes into a number of text-critical and source issues that I will not concern myself with here. Suffice it to say that he thinks ἕως ἂν παρέλθῃ ὁ οὐρανὸς καὶ ἡ γῆ likely means "never." This view is based, in part, on a comparison of both material and linguistic parallels found in the LXX. To find out each example referred to in the commentary, see Luz 265ff.
Linguistic parallels to Mt 5:18 are scanty, but Luz cites two places from the OT: Job 14:12; Ps 72:5, 7, 17. The verse at Job 14:12 is really good. Even if one translated ἕως ἂν there as "until," it could still carry the meaning "never."
You might also want to check out BDAG and Louw-Nida on this point.