The kite idea is interesting. But it seems hard to control shorthanded.
About the gennaker, it is always in use on the M32 downwind. Even when fully reefed on the main. I guess the helmsman decrease power by steering more down if the wind is strong. Also, the M32 has curved foils, which probably helps downwind in strong winds.
Since the proa can't gybe, it's going to be slower than a cat downwind. My thinking with the downwind sail is that the proa could compensate for this by sailing faster and/or lower.
About the mast, I was thinking that since the gennaker is pulling in the top, it would maybe cause more stresses than if the forces would be evenly distributed.