Ford pardoned Nixon. He hadn't been tried or convicted of anything either.
Good point. I'm quite sure he's thought of that. But I would offer Ford's pardon had no legal basis under the definition of a pardon or reprieve, and it was not tested. He'll fill the vacant SCOTUS seat, and that will set the table.
However, Ford was not directly or indirectly involved in Nixon's crimes. Given the facts surrounding the botched FBI and INjustice department "investigations" the same can not be said of Obama and Hillary.
If it can be proven to a Special Prosecutor and Grand Jury that Obama himself was actively involved and complicit in any number of Hillary Clinton's felonious crimes and activities then his so-called "pardons" could be nullified: It would be like Al Capone trying to pardon Frank Niti.
Obama's pardon could also be nullified if it can be shown that Obama ordered and or put pressure on FBI director Comey the FBI, and or the Justice Department to obstruct justice, destroy evidence, or squash their investigation of Hillary Clinton because Obama would then be an "acesory after the fact": an active participant in the crime syndicate and hence legally disqualified from issuing any pardon of Hillary or her Clinton Crime Family Syndicate associates.
Given what Trump said during the second debate, I wouldn't be surprised in the least to learn that Trump already has a team of legal experts scouring the Law for legal presidents to successfully negate Obama's Presidential pardon of Hillary Clinton and all of her surrogates should he issue them.