Before the merger

Argued here is a base text of Isaiah 1-33 to which the rest got appended. We can add here that whenever the MT diverges from this part of 1QIsa^a, 'tis almost-always best in that great Isaiah scroll.

One such locus of MT angst was in Isaiah 19:16-25. This is overall a proEgyptian oracle. Isaiah's generation had some motive to support "Egypt".

But Egypt's "dynasty 25" were Kushites. They enjoyed some loyalty in the middle Nile... but this attenuated among their paler coastal coreligionists. Assyria would eventually raise sub-kings from Saïs to push that dynasty back up the river (getting as far as Thebes IIRC). More to the point, the pericope 19:16-25 predicts Judah communities in a heliopolis. That did not happen - nor is it a desire that Hezeqiah in his Assyrian cage cared to happen.

I further find unlikely the Nubians were making promises to resettle Judah exiles as a "Plan B". Recall that the Assyrians had been resettling Israelites before all that. I won't rule out the Nubians had Plan B on the table (to counterbalance their own restive natives); but the prince Taharqa's Plan A was simply to beat the Assyrians so to keep the Jews on-site as a buffer external. In particular why would Taharqa or his propagandists tout, to Yahwists, the city of Aten-Ra (or maybe Amun to him)?

So these verses 16-25 do not belong in Isaiah 19.

Most scholars as a result see a proEgyptian oracle in the Persian time. It could be when Egypt was promising a refuge from Cyrus of Anshan; or, it could be after his son Cambyses took Egypt and was advertising settlements down there. (Perhaps not in that elephantine island Yeb; I understand the Jews came later up there and anyway was more a Khnum spot.)

The one thing I can say is that this oracle entered the text of "real" Isaiah entirely independently of its attachment to Cyrus' pet second Isaiah. Because: the second Isaiah hated Egypt. Cyrus and his heir were perhaps promising an attack on Egypt; or, at the very least, they didn't like Egypt's capacity for mischief. Cyrus' own propaganda had him as the heir to the Assyrians. Which would also make him the rightful overlord over Saïs and the Delta.