: How crazy would it be to design my own kayak?
. . .
: Plans--there don’t seem to be any that quite fit the bill. . .

Look at baidarka plans. You can get George Dyson's book or skim the websites of skin boat builders.

While these are originally meant to (skin boats many have been built as strippers by simply bending strips over the frames. Some people pull out the frames and some leave them in.

Or 'skin' the boat with thin plywood. 3mm or 1/8th inch ply works fine. Bend it around part of the frame, trace over the stringers on the frame and cut inside the line to get a panel you can glue and screw to the frame. When the glue is hard you pull the screws, fill the holes, sand, and glass the outside with light glass, or just tape the seams.
Paint the wood or coat it with epoxy and varnish.

I think you would be safe in replacing the bifurcated bow with something simpler and less traditional. On the stern you might like the traditional lines as they give good mountings for rudders.

With such long boats you pick up a lot of bouyancy as the length increases. You haven't given us any idea of your weight or height so actually getting a good fit is total guesswork at this time. Don't be too afraid of going narrower. A kayak as narrow as a scull would be very quick. And an outrigger--even a small one would make it stable.

Why a single paddle? Bigger blade and different paddling position use different muscle groups than seated double-blade paddler would use.
As the song goes " different strokes for different folks"
PGJ

PGJ