That I can think of, all the single handed spin boats I've seen have snuffers. It can be done but I can't imagine safely handling a bag launched spinnaker single handed in all but the lightest breeze. It takes us three hands to effectively douse my bagged spinnaker in a blow (two pulling in the sail and one of mine easing the halyard / tack line while my other one steers).

A square head is not really going to give you any moment by moment noticable performance improvement. It will probably shave a few seconds off your lap times but that's about it. Personally I wouldn't replace a relatively new main sail just for the added square head performance. Your gain is not worth the price.

If it were me and I wanted to predominantly single hand, I would probably add the spin and find a way to build or purchase a snuffer.