For anyone saying "he's not overturning them, he's nullifying them" it doesn't matter, there is literally no avenue anywhere in any of our foundational documents that allows for anything to happen to a presidential pardon once it has been executed. Nothing can be done to it.
There is absolutely no law on this subject. It would need to literally be built de novo, from the ground up. One would need to establish standing to even challenge something, let along nullify it. Just the issue of standing to even bring a challenge of a presidential pardon would need to be established, and ultimately the SCOTUS would likely have to take it up and decide if standing could be established. If standing could be established, to have even the slightest chance to challenge a presidential pardon one would need to prove that the president didn't actually grant the pardon. You'd need evidence "beyond a reasonable doubt" of a massive conspiracy to issue pardon's that were not granted by the president. Evidence that rises to that level just isn't likely to exist. People don't seem to understand that when a legal pathway doesn't exist you can't just will it into existence. Even the president can't do that. There is an entire process that needs to occur.