My guess is that since a self-pardon hasn't been ruled lawful in the courts, they would have had the incoming president pardon the previous administration. That way there would be no question. This would have obviously been a problem for Hussein because Trump would have never gone along with such a plan, forcing Hussein to pardon himself and those around him.
There is no such limit on a presidential pardon. A president can pardon someone of any federal crime, including treason.