With things being 'non-standard' I would check a few things before looking to the finer points of tuning. Check the hulls are parallel, all foils are in good order and the rudders are correctly set up to lock down in line with each other(sight leading edge to leading edge) and check the alignment(toe-in).
The Inter 20 carbon mast is designed for a heavier boat and crew weight and consequently is stiffer than you may normally choose for your configuration. I am sure the Inter/Nacra 20 sailors can give you a base setting for prebend. You may require more prebend than base with your setup. Was the sail cut specifically for the mast/boat configuration? If not get a sailmaker to check over your rig. He should be able to establish any changes needed to the luff curve suit the mast. Additionally a softer boltrope and/or powerful downhaul may help compensate for the stiff mast.
With the platform straight and the sail matching the mast you can then look to balance the rig. If the mainsail has not been carefully matched the drive from the rig could be anywhere in relation the lateral resistance.
Mast rotation - With no jib to deflect air around the back of the mainsail a unarig is much less tolerant of poor trim. You just have to work at it to find the sweet spot.
Don't trapeze too early to keep the windward hull out and be very wary of trying to measure performance against F18's if you maybe in their dirty air.
No easy fix really. Things might not be far off, but with non-standard you just have to work through the whole boat.

Cheshirecatman