A couple of points from an A-231 perspective:
"The manual (pg 23)shows diagonal supports, but does not show how they terminate on the lower end. Is there any additional structure not pictured in the drawings? "

Construction detail from A-231: A-231's diagonals terminate at the surface of the vee berth.


"The plywood "bulkhead" is tabbed to the hull ONLY and not to the under deck or the coachroof sides or overhead."

Construction detail from A-231: the original mast beam was buttered to the overhead with thicked polyester (the ubiquitous blue stuff). I did the same with thickened epoxy when i replaced the beam.