If you don’t believe Moshiach could come from the dead you are a kofer in Toras Moshe and have no chelek in Olam Habo!
As I believe Sechel Hayashar has posted, before the Rebbe was niftar, nobody considered the possibility that moshiach could be a dead person. Digging up supposed shittos that moshiach could be a dead person was done because of many people in Chabad being unable to accept reality.
Saying he is “b’chezkas moshiach” is the distortion of Torah. Just read the Rambam, and hold the sefer straight, not upside down.