Funny you mention that. Because those Jews (not all of them, mind you) that did not accept the Messiah did try to change the book of Isaiah. The mental gymnastics about the "Almah" translation continue to this day.
https://jewsforjudaism.org/knowledge/articles/isaiah-714-a-v...
As I understand it, a number of people claimed to be the Messiah in Jesus' lifetime (and before, or since for that matter, including today). I don't think Old Testament references to the Messiah are all that meaningful as such for this particular discussion. Whether Jesus is or isn't the Messiah is of course a matter of faith.
Yes, they imitated and used the "concept of Jesus". That's why I think St. Paul did not invent it.