1) Mindless undead will not care about you if you, yourself are undead. They might be ordered to attack you by an outside influence, but they will not do so instinctively. Intelligent undead either will or will not attack you based on their attitude towards you. That Mohrg will likely despise you for keeping it imprisoned all this time, even if you feed him corpses. He's still a prisoner.
2) If you can get some sort of Mythal-based Hide From Undead effect going, that would do the trick for the Mindless undead. Intelligent ones would get a save, of course, but those can be reasoned with (or simply ordered to lay off the peasantry if you're significantly stronger than they are). Greater Shadows, however, have an inescapable craving for Strength, so they're liable to go out killing anything and everything they actually see if you don't keep them well fed.