Would that really be the "only" heresy one could commit while praying?
Your post is fraught with illogical statements of that kind.
I don't know anyone who prays to "dead humans" - though I do know many people who pray to a human who was once dead. His name is Jesus of Nazareth.
Since that same Jesus is also the living God as well as a human being who suffered death, I'm not sure why you believe He lied to His mother, Mary (I assure you, by the way, that she exists - there's no need for scare quotes around her name), about the gift of everlasting life.
The quotes are because those that pray to “mary” are in reality praying to “the mother of god” or “the queen of heaven” who are none other than Semiramis/Ishtar.
Jesus was victorious over death, so he is not a dead humam. Aside from Enoch and Elija, no other humans have yet been victorious over death. The “first resurrection” AKA rapture, is when the “dead in Christ” will experience that victory.