How about getting a Nacra 6.0 hull as mainhull? We justed switched beams between a 1989 Nacra 5.5.18 sq and a 1992 6.0. The 6.0 is now 3.3 m wide and the 5.5 down to 2.6 m.

Not sure about the non-sandwich 5.5 you have, but the later sandwich models have the same distance between front and rearbeam than the 6.0 making it a very easy operation as even the tramps did not need any modifications.

Regarding your trimaran project, the overall appearance of the tri would look much more harmonious than what you achieve with a tornado shaped mainhull.

Maybe you can find a damaged 6.0 hull so it would safe quite some money and time.

Carbon tubes could be a good idea to join the three hulls.

Can't really comment on the proposes angles and height differences. If you go with a 6.0 mainhull (and the distance between front and rearbeam is identical with the 18sq), would probably connect the three hulls with long aluminium tubes and go for a ride. This experience would probably give you a good feeling how much you want to cant the outside hulls.