I would say for operational simplicity, the spacebar to start recording and second tap to start playback would be optimal, then maybe return (or something easy to get to) for Stop of the loop...

I don't think that any CS's ever had input quantization. They basically left it up to you to be accurate enough. Might be nice as an option (with 1/4 through 1/16th resolution), but on the whole, not really important.