Martin,
Sorry, I'd have replied earlier, but I've been at sea for the past week. Only just getting back into rather feeble internet reception.
I think you should certainly be putting some camber intoyour lower panels. Don't be put off by the reports from people with high aspect ratio schooner sails, where I do think there is a difficulty in getting them to set well. On a lower aspect ratio sail, it can be done.
Arne's sail planform is basically H/M, with the uppermost sheeted panel made in a different shape, its uppermost batten set at a higher angle. This reduces the size of the top two panels, which is what you're looking for. I think you'll get an effective, simply made sail by going this route.