Yes you have it pretty accurately laid out. My '73 didn't have any reinforcing layer of aluminum, and I don't recall any steel angle that helped support the hinge for the bumper trunk, but who knows, it may have disintegrated by the time I took it all apart.
1) The solid piece you are talking about is the rear hold down plate, right? Yes, it is "L" shaped, but not at a 90 degree angle. There are three or four big bolts that hold the rear C-channel, subfloor, hold down plate and rear most cross-member together. No welding on my trailer at least.
2) Correct this issue by building a piece of flashing out of aluminum that essentially takes the place of the reinforcing aluminum you show in your diagram, and the base of the "L" extends to the left (in the diagram) and fits under the aluminum of the bumper trunk so that water that heads toward the subfloor gets redirected toward the back (bumper trunk).
3) Again, I don't recall having this angle in my trailer. Seems superfluous.