The new president (Biden) could simply pardon the outgoing president and then it would just be a simple case of presidential succession via the 25th Amendment.
You may need to get a real lawyer to explain the difference to you but those cases are procedural deficiency cases, not eligibility cases. Miscounted votes; voters in the wrong place etc. wouldn't viatiate the outcome once established by whatever political procedure is in effect to validate it.
Zero's problem is eligibility to hold the office. Same as if he was only 20 years old when elected--not eligible to hold the office. Assuming he is not a natural born citizen, nor for that matter a citizen at all, he isn't eligibile and isn't President--that simple.
Pardon by Biden? To do what? Wouldn't affect validity of his actions in office--to the extent acts were invalid they remain invalid unless implemented by acts of a real President. For example, Biden could renominate Sotomayor or Kegan; try to get them confirmed again. Anything is possible.
There are criminal acts involved by a number of people--maybe even including Biden. Would Biden pardon the perps? Maybe but probably not.
Wouldn't surprise you if at more than one point in this dance, Biden was looking at his own hole card.
What was Richard Nixon pardoned for? He was only named by the Watergate Grand Jury as an “unindicted co-conspirator.” He had not been tried or convicted of anything.
When a president issues a full pardon, it covers all contingencies. When George H.W. Bush pardoned former Secretary of Defense Caspar Weinberger it was before Mr. Weinberger even had gone on trial in Iran-Contra. The pardon stopped the trial cold.
If Obama was in serious legal jeopardy, he would resign the office (my preferred way to get rid of him) and he would accept a full pardon for any and all crimes that he may have committed from his successor just as Nixon did.