Here is what I understand about the process. If any of this is wrong or technically incorrect, feel free.
Nixon avoided jail when he was granted a full pardon by Ford and he was not impeached prior to his resignation.
A POTUS (Obama) is only impeachable while he or she is POTUS. If he is impeached and convicted, he can still be charged in a court of law where Congress has no vote (trial by judge and jury).
If Obama is not impeached, he can still be charged post-presidency if certain criteria are met and if there is sufficient evidence to warrant the charges just as Nixon would have been had Ford not pardoned him.
I can’t see any situation in which President Romney would pardon Obama.
Hmmm