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.
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.
Correction: They didn't want to risk destroying any of the supplies inside, so tactics such as shooting the lock or destroying the door were out of the question. It is a much safer tactic to simply unlock the door, and not risk what they came to plunder. Obviously, something so simple and small as the lock leading to Mercedes was an oversight.
Correction: "Moves around as if unhurt"? He collapses to his knees after the initial stabs, and almost completely falls over after his mouth is cut. He is then seen staggering out of the doorway, stiff and angry. He simply is acting like it doesn't hurt in front of his men, so he doesn't tarnish his image, but it is clear that he is hurt very badly from his initial reactions. Not to mention, shock from the injuries would also set in and take away some of the pain.