I admire your tenacity !! I didnt know that the 'glass house was lined post mold with the interior ply.How the heck did you find the screws? Must've been from the inside! The side panels on the 58 are very much visibly individual panels with battens behind and trim covering the joints. Even still, most joints, edges, and corners are beat up, so replacement is the order of the day. It never ceases to amaze me at the low quality of the batten-paneling job that the factory did. Its REAL sloppy work. If the did it in Chicago, I would call the craftsmen SHILOCKS !! When I redo them, it will be with removal in mind to provide access to blowers and electric etc.
Right now, just the lower panels need R&R. I think from the sill up can be salvaged. FWIW-- I just bought a jar of SAVOGRAN oxalic acid, poured about an ounce in the crapper and it instantly removed all the rust that was otherwise impossible to remove. That was great! The mustache is next! ws