The Jewish tradition is that Hezekiah and his men wrote/compiled Isaiah (Bava Batra 15a; Talmud). We know that Hezekiah compiled some Proverbs (Pro 25:1). We also know that other works, like Jeremiah, seem to compilations/anthologies rather than singly composed piece of literature. Jeremiah, as you know, is ordered radically different in the LXX than in the MT. The content is roughly the same, just a different order.
From this evidence, I wonder if the critics are right in this ONE respect: Isaiah 40 represents the start a different and disconnected section from chapters 1-39. Does this warrant the conclusion that these are two different authors? No, not in the least. It is equally plausible, and sound doctrinally, to argue that both sections are written from the same author. Only, they represent different works from the same hand. By analogy, David too authored many Psalms differing stylistically in tone and topic. Yet, these were all compiled into the single book of Psalms. Where the analogy breaks down is that we have Psalm headings to enlighten us as to the different authors.
Further, seen as differing works, this may provide some insight into the balance of Isaiah’s ministry. At times, his ministry focused on judgment. And so his writings focused on that. At other times, his ministry focused on comfort and hope. And at those times, his writings followed that. By way of analogy, it could be something like the apostle Paul. To Corinth, he wrote judgment. But to Philippi he wrote Joy.
Here is where the rubber meets the road for this question. In some senses, 40-55 is disconnected from the earlier chapters. And if my above reasoning holds true, then it may represent a different work (albeit from the same hand–the historical Isaiah). If so, then it can be preached as its own separate unit in much the same way we preach Psalms separately. This does not mean, however, that it is completely disconnected from the rest of Isaianic literature. Many of the themes, such as the sovereignty of God, are the same. He also enjoys some of the same literary genres, like the covenant lawsuit (chs 1 and 41).Therefore, it is appropriate to connect it to earlier sections. In the same way that we connect Paul’s epistles (see second paragraph) so we connect these sections of Isaiah (remember one author, different sections).
How do we preach it in connection with the redemptive story line? This chapter easily fits into it in many places. You can trace the Messianic line through the ages within this chapter. You have the original setting of the Assyrian attack in 722-ish BC where the kingdom almost fell. This gives a message of hope in that crisis. You have the Babylonian captivity (c. 600 BC) and return which is the near fulfillment of these words. And most of all, you have Christ the king whose messenger goes before him preparing his way.