This is just a thought or two based on having to work on a Lionel aluminum passenger car of the same period but 21" long. I hope I have understood what you are still having difficulties with and pardon me if I haven't but here goes:
The lower/main section of the interior is likely to be fixed to the metal frame by some very small screws the heads of which are on the bottom of the frame. In most of the cars of this kind I have worked on these screws are actually under/concealed by the silver frame detail assembly, which itself is just screwed and/or clipped to the metal frame. If you have taken off the trucks and the frame detail you should see any screws of this kind if they are there. Removing them should free the main interior from the frame so you can get it out and re-attach the passenger figures.
(From memory the dome interior is fixed to the underside of the dome or the body shell itself. The wiring to the dome light strip should be attached to one or both of the main section light strips. As I think you have already found some of the lighting wiring feeds up through or at the ends of the frame and has to be disconnected so you can try sliding the frame.)
If you don't detach the main interior from the frame you have to slide it out with the frame. The last time I did this it was a nightmare until I got the bright idea of shooting some WD-40 into the C-shaped channel that the frame slides into. I was wary of trying this because I did not know whether any overspray would be corrosive to the car's painted finish. It wasn't although I wouldn't be sure that the same applies to the polished aluminum surfaces of your cars. In any case, if you try this use a WD-40 canister that has the small plastic tube fitted to the aerosol nozzle or try applying it to a Q-tip and using that to get the lubricant in the channel. After a few minutes the frame slid straight out. I did find that one end of the channel was pinched and so the frame was also slightly stuck at that end.
Just about every Lionel or K-Line (or GGD/3rd Rail) streamline passenger car I have taken apart has the screw/channel configuration described above. I hope this helps.
P.S. Is this the car you are working on? The frame/interior assembly looks similar to what I have described: