The best way I can analogize it would be the hexenwolves. Phillip is Sophos' "Demonic" Co-Pilot in that while Sophos could take control of the body, it would be with Phillip's consciousness either asleep, regressed or dormant, but Sophos would still be running off Phillip's subconscious. All the involuntary actions would still take place, and the subconscious way most people walk, talk and breathe (you don't think left, right, left, right when walking unless you're a soldier) would still be in effect. Like the captain of an international flight letting his Co-Pilot watch the instruments while he sleeps.
As for undefyable orders, I'm not sure. Sophos isn't aggressive, so Phillip remains the dominant personality. The Rules of etiquette might paint Sophos as a permanent guest, so that could influence things.