Sounds like you've thought about it more than I had.
Split like this, two ways, the height of the storage would be cut in half.
You could have something like the tailgate area above pushed back to mimic the trailer tail. The bottom half could swing 225° to put the shelving on the corners when it's open. That would put the hinges in the new construction.
Insofar as the Trailer Tail—it is a box cavity (for stowability). A flat truncation (flat, concave or convex but with a sharp separation) will perform better, though not as well as a 'later' truncation or full boattail.