Final proof? Easy! In mathematics we have a simple property that says a = a: the reflexive property. Logically, you can expand this out so it applies to anything.

"I am not myself" is clearly wrong if "I" and "myself" are the same, just as "dead" cannot not be "dead."

The primary essential property of "Dead" (or anything) is that it is itself. Dead is Dead.

This is similar to the definition that statements are true or false (excluding paradoxes). They cannot be true and false, nor can the be neither true or false.

Dead is dead. The question is "is it all dead, or is only the material part dead while the mind/spirit lives on?" This one is harder to prove. I willn't try here.