The time for the player to make input is when he announces his action, not after resolution. If the player says, "I take his head off with my sword, or at least try to" and the dice decree a kill, then I say "His head makes a gentle arc, bounces twice, and rolls to your feet, a slightly surprised look on his face." If the player says "I attack" and the dice are favorably disposed, the I just say "Okay, he's down," or "You take him in the chest and he folds." The "Mercer" things you're describing, or whatever it is, is just weird.
...after a few more moments of thought this is just another application of "Players control their character, GMs control the rest of the world." If you're giving players narrative control of the world, then you're starting to move out of classic RPGs and into something else.