I got your mail and here's the answer.
Sailing the M18 with double trapeze is possible and the platform can handle it. I've sailed my A-class with a combined weight of 125 kg and single trapeze and noticed that the mast top flexed too much and you lose a lot of power from the sail. Mitch Booth made a F18ht out of a M18 by adding a cut down M20 mast, M20 centerboards and about 40 kg of lead in the main beam to meet the minimum weights. The platform was ok but the boat seemed to be a bit low in the water.

If you keep the crew weight below 140 kg you will probably get it high enough in the water but you need a stiffer mast! Try a F18Ht mast and sail maybe?

/hakan