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.
I believe he can be pardoned at this time. Bush won't do it though.
So you are saying that a pardon is OK before a trial starts but not while it is in progress or after a conviction until all possible appeals have been resolved? Do you have some reference for this novel legal argument?