Corrected entry: What possible reason did Mercedes have to give the key to the rebels? An ancient padlock and a flimsy wooden door wouldn't slow the well armed rebels by much - they could have just as easily destroy the door or shoot off the lock. The fact they had the key, however, reveals to the captain there is a traitor in his midst. No remotely competent rebel force would do such a mistake - and this force is more than competent: just having survived five years past the defeat of their side in the war attests to that.
Corrected entry: After Mercedes sticks him with the knife in the back and shoulder, the captain moves around as if unhurt (except for the mouth cut), despite the moderate damage suggested by the knife cuts.