1 - rear sill is hard to repair, but the only choice unless you can find a good used one
2 - that's the jack rack, but I've never seen it there, either under the front of the driver seat or under the hood
3 - if you mean the license light, the one you have is similar to the 57-59 assembly
shown here, but on a 55 it should be
the earlier style, like the AD - must be cuz of replacement doors
4 - yes, that driver seat is cobbled up, the pass side is the original driver seat - used might be found, they were the same in AD years and I believe in the base sedan delivery
6 - yes, that bottom latch was added
and yes, the floor is one over-size sheet of 3/4 ply with the skid strips inset, special order if you want to replace it
Bill