Autopen wouldn't make it void, the issue of autopen signatures has come up many times and court cases have not held them to be invalid. The real question is whether Biden actually decided to approve the pardons or someone did it without his knowledge. I don't think this is going to go anywhere because to void the pardons you would need to prove Biden didn't approve of the pardon. That would require him to both remember and admit that he didn't sign it or possibly a witness to say Biden didn't approve it. Even if he they signed it without him knowing I don't think anyone is going to spill the beans.
Show me where anyone said just using an autopen made anything void.
it can be argued that the founders never anticipated a device that someone other than the president could use to sign his signature.
In most government offices, the standard is to place the signature information (signature block) that belongs to the actual signer. Then below that it states they are signing for the actual person in charge. Basically they are authorized to act on behalf of their boss, and the signature block properly shows the actual signer as well as who should have signed it had they been available.
I guess the President can't delegate anyone else to do his constitutionally required duties, so it's not surprising you never see something signed by the VP or WH Chief of Staff saying they are signing on behalf of the President. What does need to happen in cases where an autopen is used, especially where the Constitution explicitly reserves that authority to the President alone, is there needs to be documentation either in written voice recorded or video recorded form that proves the president authorized the signature and was fully aware of the contents of what was signed.
Ask Joe today to explain all of his last minute pardons, he's going to not remember. However, he will pretend to remember. That's what people with dementia do. It's too embarrassing to admit you can't remember something as important as a pardon. It's no different than someone with hearing loss nodding in agreement or saying, "Yeah" or "Okay" to something they didn't hear you say. it gets embarrassing to ask the same people to constantly repeat what they said.
Short of notes, recordings or other tangible evidence, I'd say Joe can't prove he was aware of the pardons.
The fact that the pardons are preemptive for any crimes they may be charged with in the future makes the entire situation 100 times worse. Most pardons are very specific as to the statutes associated with the crimes being forgiven.