Bruce

I think that you have two ways forward here, roller furling, as you say, or a snuffer system.

Roller furling will be easier than the current snuffer systems since to douse the spin into a snuffer will require loosing the tiller, allbeit only for a few seconds. No problem in 15 mph, but more dificult in 25. The downside with roller furling is that I think you'll take a big hit on upwind performance.

The very best way would be a snuffer system that can be operated 1 handed, for this you need a pump up and pump down system. Complicated, but you could hide the stuff under the tramp, and it would be quick, easy and only require one hand to get the kite up and down. If your looking to develope the best I'd go this route.

15-17.5 sq m should be the right kind of size, but I wouldn't recommend any cleats on the sheets, just make sure the ratchets have plenty of wrap.