So my idea for the headless corpses is to turn them into phantom armors (https://www.d20pfsrd.com/bestiary/monster-listings/undead/phantom-armor/phantom-armor-guardian). Since the body is destroyed in the process of creating them it shouldn't matter that they're missing heads. They're intelligent undead and understand common, and while they can't speak they should still be able to either A. write down responses on paper if questioned, or B. be instructed to retrace their steps from before they were killed.