1) On the contrary, I think all the absurdities he's spouted during his time in office, would only BOLSTER and add credibility to any claim of diminished mental capacity. No sane person who is right in the head would ever utter such crap publicly.
2) Again, as I said previously, it would be better than growing old and frail in prison. Even if they they take away most of his money and real estate holdings, he knows he'd still be wealthy by most standards. Plus, there's his Presidential pension. Given the chance to live in a comfortable manner that most people cannot afford, vs living in a cell, I think he'd do whatever was necessary.
Even if it means making himself look like a certifiable Looney Toon.