If Trump makes an early exit and Pence pardons him after being sworn in, it’s game over. While ethically odious, such a pardon would be constitutionally sound and legally binding. Assuming the pardon is sufficiently broad, attempting to charge Trump with federal crimes would be a nonstarter. Maybe investigations could be completed, maybe the case against Trump could be set forth in an indictment against somebody else (the Trump Organization, the defunct Trump Foundation, one of his kids or employees, the campaign committee, etc.), but Trump himself would escape criminal accountability for any federal crimes committed prior to the date of the pardon.
But if Trump purports to pardon himself, all bets should be off.
Federal prosecutors should view a purported self-pardon as an invitation to file any charges warranted by the facts, full stop. Criminal indictments would not only hold him legally accountable, but also would vindicate the principle that in the United States a president is not a king, cannot be his own judge and jury, and does not have the constitutional authority to pardon himself. There is no precedent for a president pardoning himself, constitutional scholars are divided on whether it is possible, and the only way to test a purported self-pardon would be to file charges, let Trump assert a defense that his self-pardon immunizes him from prosecution, and defeat that defense in court.