So President Ford's pardon of President Nixon was invalid?
He hadn't been found guilty of anything; that was a pre-emptive pardon. Some people thought it wouldn't have stood up.
"The President shall be Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any subject relating to the Duties of their respective Offices, and he shall have power to Grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment."Nothing about "the process has to be complete".
I believe Nixon was pardoned of any "possible" convictions that would have come about in a future trial. he was never tried and couldn't be because he was pardoned in advance. I this case the trial went forward and I suppose has to be completed including all appeals before a pardon can be issued.