Hi Kloudie,
My boat has not a single one of the screws you point out in the picture. There is exactly one brass screw visible from the top, and it's right at the corner closest to the galley. There are screws pointing upwards through cleats into the plywood that are accessible through the access covers, but there appear to be more between the tank and the wall, and there is no way I can get my arm in there. I can't figure out how they put this together in the first place. My only guess is that they assembled the complete settee (top, side, and end) and somehow attached it to the hull liner in one piece over the tank.
This is a project for next winter now; the weather is too good now to spend time working on this. I think it'll probably come down to cutting another access port, now that I know the likely location of the crack.
As always, thanks for all your help.