Wood frame repair - advice needed

In my ongoing battle against entropy's effects on my TM, I just finished replacing the completely rotten (and largely nonexistent) vertical wood frame piece on the latch side of the door (upper shell). During this little diversion, I discovered that the wood frame on the rear end of the upper shell (same area) is also largely gone. (see pics)

Where advice would be helpful is in figuring out how to access the back side of the shell to insert screws. The arm pocket has screws that hold it in from both sides (interior & exterior), so these will need to be replaced. When I did the door frame, I could get to both sides because it was accessible. This part won't be easy to access because of the shell overlap. Even with the rear shell completely down, I still won't be able to get to all of the "inside" screw locations. All I can think of at the moment would be to drop the rear shell, and then unbolt both rear arms from the front shell to allow it to pivot down at the front & up at the back - thereby exposing the interior side of my entire work area.

Any input very much appreciated!

Steve