Sorry for the third post – I tried looking up the said Iggeres and found it here:
https://chabadlibrary. org/books/admur/ig/1/85.htm
I don’t have time to read through it in depth, but a cursory browsing shows that there are MANY MORE sources which can be brought up as questions to the Rebbe’s claim, and the Rebbe goes through all the available sources in this letter to back up the claim that indeed all Yisroel will be redeemed, including reshoim… If you have the time it’s worth a look, and I’d love to hear your thoughts on the letter if/after you have done so.