Mike, I'd thought about the 3/4 ply not being an actual 3/4, and thought about shimming it with a piece of 1/8"... that would make it a bit over 3/4, so 3/4 insul migh not completely fill the void, but with enough adhesive piled on the inusl, it could work...
re the weight. a 3/4 ply weighs 75 pounds.. figuring 2/3rds would be scrapped, the wall would weigh 25# which is pretty acceptable...
2 things... first, I would consider ceps on the exposed ply edges, if not all of the frame to stabilize it... second, door frame of 3/4 pine for latches and hinge mounting....
I'm guessing nothing has to be over 2" wide, so a lot of material would be removed and a nice curve could be attained.... the roof would probably be nail free, glued down with strapping to hold it in place... do not know what type of glue to use if the frame has been cpes'ed... maybe epoxy would be the adhesive of choice. If it had been cpes'd for more than a couple of days, it would have to be roughed to for the new epoxy to get a bite...