Hmmm, what kind of battens do you have up there? Tapered and well tested? If the batten leech ends are too soft, you can get some flutter in the leech and more twist than you wanted in the top. Have the top inverted with full downhaul and strong wind?

If your mainsail goes slack in between battens in the leech in light winds, I would be tempted to speculate that there is too much luff round for your mast setup. Easiest and fastest way to test it is probably by putting the boat over on the hard and set mainsheet and downhaul, then adjust diamond tension. You could do the same on the water, but I like to get in the ballpark with the boat on its side and on the hard. Of course, that theory goes against your twist problem.. I would have put the boat over on its side and spent the time to find out what is really going on.