Regarding lack of rivets in the vertical stiffener.
If the vertical member suggested by the deformation in 2-2-V-1 is only 3/8” wide, is that wide enough to rivet to?
If made of metal, it is possible that part of that 3/8” width included another leg( “L” shaped) or two legs (“U” shaped) and those legs took up some of the width which would not leave much for the rivet seat. Also the hole for the rivet would not leave much area of remaining metal around the hole.
The vertical stiffeners in other Electras appear to be notched to allow the stringers to be continuous with the two legs of the stringer “U” shape set into it.
Assuming they re-installed a vertical stiffener Station 307, and it was only 3/8” wide, and was notched to allow the stringers to continue, it could be that the vertical stiffener was solid material, maybe even plywood(easy to shape a curve and possibly locally available) and therefore could not take rivets. Maybe part of its purpose was to help “form” the curve in the middle of the opening until the assembly was riveted elsewhere and since the original circumferential structures look like complex shapes, plywood may have been an option they chose as a substitute?