Regarding the leeboard position, I had intended the rear ama beam to be well forward and bisect the cockpit just abaft the foreward seat. This was for two reasons, firstly it would put the rear beam in the ideal position for the leeboards and thus reduce the number of fixings which must be made to the hull, and secondly because it would leave the rear of the boat unencumbered by beams to allow the aft occupant to paddle (and possibly fish) without restriction. Obviously it would require a more manly rear beam but I think the advantages would be worth it. I've seen single-beam kayak trimaran which worked well enough.

As for the rig, I had intended to use a very simple junk/gaff hybrid rig with the boom (in reality just the lower of four battons) braced to the mast at a fixed 90 degrees from horizontal and all four battons/yard tethered sliding to the mast at their forward end so that the luff does not extend past the mast. This should allow simple sail handling with only a halyard and a sheet needed to set, reef and sheet the sail (reefing being a case of simply letting out the halyard with the junk rig which I do like the idea of in a kayak). There would be no need for parrel, lazy jacks or downhaul, plus the mast can be kept well in front of the fore occupant whilst keeping the centre of effort abaft and roughly over the rear ama beam. It would also allow each panel of the sail to be cambered without the mast interfering on one tack and could in theory allow a foresail also though I doubt I'd bother. Both lines will be run to opposing sides of the rear ama beam so that either paddler can operate them. However, since seeing the CLC Sail/Ama plan/kit I'm thinking I'll just use that. It's not far away from what I'd planned and has the enormous advantage of being tried and true. ;)

For the rudder I will design it deep and with a slight forward sweep to give some balancing effect and reduce effort, though as I will be using foot controls I do want some self-centring effect.

As for the Trika 540 I had not seen that and I thank you for bringing it to my attention! It does seem to tick a lot of boxes though would be a more involved build. It's a good bit longer too, which of course is good and bad.