I'm pretty certain pardons narrowly cover a specific possible crime. He wasn't pardoned for Watergate. I mean, if it turned out he killed someone and buried them in a shallow grave in 1972, the Reagan pardon wouldn't help him.
But again, the idea that this guy would be prosecuted is silly. Even if he wasn't senile.
A lawyer might figure out if it really is full or just full in regards to that one part.