Holmes at the Reichenbach falls might have direct relevance to the supposed resurrection of Jesus. I don't know if it has any particular relevance to determining who Isaiah was referring to when he spoke of "The Suffering Servant". The nation of Israel certainly existed (to whom Isaiah is indeed referring) and most of it was led into captivity in Babylon, events which have particular relevance to this particular OT book, whether you think it was written by one scribe or three.
Of course, Isaiah believes he is speaking for his 'God' here, and I know that upsets you. But I don't why every question on these matters has to be reduced to "there's no proof that God exists" (even though I certainly agree with the latter).
No matter what anyone thinks or believes, why should there any more relevence/importance afforded to the supposed thoughts of Isaiah than Holms, unless of course it can be established that Holms was a real person living at the time and place the writer of the story says he was; nothing particually deep about that, or likely.
Assuming the above, why is it any more important to discuss the thoughts of Isaiah, or any other supposed religious figure than, purely as an example, Holmes, unless of course some of the verifiable stuff is found and since that is so unlikely to happen, why should I, as you suggest by the tone of your post, be expected to take this sort of discussion more seriously.
(No spell check on my tablet, so the spelling is the best you're likely to get from this direction).
ippy