I'm aware of the EO that gives POTUS the power to call up former military but that makes no sense in Mueller's case. He's already in the thick of it without being recalled by the military. Plus POTUS loves the military and I doubt he'd like to cast any negative light on the Marines by trying Mueller as a Marine. This will be a year to celebrate the greatness of the Marines.
Mueller is a bad actor who has been turned in return for avoiding the death penalty IMO. I think the "job interview" was where Mueller got to see the sealed indictment they have ready for him while they explained his options to him.
He chose life which means he'll end up as a coward, and a traitor to everyone…..his country, the govt., the deep state, his peers, the Marines and everyone who ever worked dutifully under him. That's my take.