I worked in Washington for 11 years. It would be absolutely impossible to keep such a fraud a secret for more than a few hours: The place is full of leaks and leakers. Conspiracy theorists, give up!
I agree that he’s most likely dead; it would be, as you and others have written, nearly impossible to fake, and the risk of being exposed too great. And, as Dave Hirsch pointed out, it’s too early in the election cycle for that.
I am curious, though, if you consider it a remote possibility that he’s been captured for information. It would make some sense, if he would be caught alive, to feign his death. He still couldn’t release a tape of himself, and only very few people would know the truth. His death (or claim of it) is, IMO, safer than his being captured alive.
I don’t think this is likely, but as an “insider”, I’d like your opinion.