silenced killing should work. just keep trying.
Reverse pickpocket better armour, save prior to doing,if it fails and you get caught,reload.
Pvt. Halford in the Guardian Caves has an MP uniform for some reason - he's easy to kill since he's alone, and he'll probably die on his own if you don't keep a close eye on him. If you want him to live and want his MP uniform, plant better armor on him before healing his leg, then go and come back.