Wouldn't the whole thing be easier if "closing the loop" didn't involve sending the hitmen back to their younger selves? You have a small army of assassins--just send Bruce Willis off to his younger self's Looper buddy. No interfamilial (as it were) recognition, no one getting all sentimental about their mortality, business runs smoothly. Bingo bango.
I hope my future self is listening. (I am, buddy!)