Deutero-Isaiah

Question for the scholars: How strong is the evidence for multiple-authorship of Isaiah?

Exodus is clearly written by an outside author, since the Song of Sea has archaic Hebrew compared to the rest of the text (I don't know Hebrew, but this is my understanding).

On the other hand, multiple authorship for Paul's letters seems quite weak, since early tradition ascribes all thirteen letters to him. While the pastoral letters do cover different topics, the audience is also different, so I wouldn't be shocked if there are differences there.

As for Isaiah, it's clear that the different sections address periods separated by centuries. I don't think you will convince most Christians on those grounds since the claim is that Isaiah can see into the future.

So what's the evidence? Is there something that I'm missing? Is there an appreciable difference in writing style in the Hebrew that a translation doesn't show?