If the “except in cases of impeachment” clause doesn’t work, and not much in that document works with the current resident, then there is the charge of treason - that would be guaranteed to prevent any pardon, IF, it can be prosecuted to the fullest, and I emphasize “fullest”, extent of the law.
Yes, in that case, there would be no way to overturn this decision... it would be what one would certainly consider, “final”.
The optics of a pardon, would kill the demicomme party. If obambi tried to pull a pardon all the bastards cuse I’m having a petulant frenzy, the fed gov would,lose control of the Nat Guard and any chance of ever influencing internal criminal investigations. The FBI would be terminated. I bet the secret service would walk. You can get away with a lot as potus, do something treasonous and there is no place on earth you’ll be safe.