not sure what your question is as it applies to your dilemma or if there is one, if you are putting in another beam(size,construction,location) why not simply lay a strip of plywood where desired and weight/clamp it down,it will bend to fit the shape of the deck. If you are thinking of making a big chunk of laminated beam like the deck beam under the coaming don't bother that would be overkill. If you are thinking of making recessed hatches then the whole issue is moot, the hatch recess and spacer become beams ( 4mm deck plus 8mm of spacer is a lot of cross-section) especially if you make the spacer extend to the shear clamp,not as one big section of ply but as though a 1 1/2" strip was extending from shear to shear blending with the hatch hole spacer.

So with the regular rear hatch you could attach the deck, cut out the hatch hole, install the regular 4mm "beam" strip,sand the strips edge round the lay glass between aft bulkhead and over the strip. plenty strong.

If you wanted to complicate the process more but realllllly do the job fancy and light you could glass the underside of the back deck(between bulkead and a few inches aft of aft hatch strip) BEFORE nailing deck on. The LT back deck doesn't bend much and if you do it as the epoxy is tack free but not hard it won't be a problem nor affect installing the deck,probably wouldn't be a problem if it was hard but why chance it?. Then when you cut out the hatch you'll have an already glassed hatch cover and back deck then when you glue in the spacer and recessed lip the deck will be strong enough for anything.

As far as how will the butt joint look with two pieces of plywood hitting at different angles? twisted! that's why you put the but plate underneath and clamp them into the same plane,,,glue dries and ta da, smooth transition. Same thing with the coaming, lots of clamps, wood bends, glue cures, wood is now part of something bigger. onto the next thing, don't worry it's not rocket science.