I tuned him out when he backed out of an engagement for St. Judes, iirc, because the benefit had something to do with Trump or his family.
At least a couple of years before Trump announced his candidacy.
Yes, Seinfeld sent a donation in lieu of his appearance.
Maybe, but he has gotten in hot water for being anti-PC multiple times.
He may harden his liberal stance and apologize, or he will be shoved into conservatism by resentment.