Can I disguise myself with the armor of a guard I just killed, or do they always know who I am? Could I pass as a hold guard and get away with anything I want?
Disguises do actually work, but only to an extent. I was watching Jesse Cox play skyrim, and in the Thalmor embassy he attempted to disguise himself as one of them. When he approached some Thalmor, they didn't immediately attack him, and they said that they didn't recognize him, and they wanted him to come so they could see his face. They would have probably attacked after a few seconds, if he didn't just then get attacked by 2 dragons.