Caulk around window is not normal. Normally the window is removed and has butyl putty tape applied to the flat flange around the edges and is then pressed back into opening and re-riveted.
Yes you just drill the rivet out. Most times one can use a drill bit the same size as the rivet shank. Head will pop right off as you go through. Often sliding up the drill bit. If the shank size bit goes through and head is still on then go up a size or two in drill bit and carefully drill to the point of just through the head. Drill bit grabs head as it breaks through and the rivet head breaks off. You just don't want to drill all the way through and make the hole larger.
People paint ensolite inner liner. The liner provides insulation to prevent condensation and noise carrying through the walls. As well as the walls being translucent without some cover so one would put on a magic lantern shadow show for the campground. Most embarrassing if one is laying on the bed struggling to get ones speedo pulled up, or those too tight sweat pants :-)
The liner may have originally had a white foam tape over the seams. Scamp still
sold it a few years ago. Stuff they make today doesn't stick all that well without some additional adhesive. 3M spray 77 if applied to both wall and tape works, as does contact cement. Spray adhesive is a hassle to apply to the wall, I cut a slotted template of cardboard. It is great to apply to tape. Went to brush on contact cement for more control. Seems to hold better too.
Some people just caulk the liner seams, one can dab with a sponge to give the wet caulk a texture similar to the rest of the liner. Not sure of best choice of caulk product. Paintable if one is going to paint, needs to remain pliable. Enough people have done it that a search of the site or posting a specific question on caulk for ensolite seams would get you plenty of first hand information.
The roof can't really take a persons
weight. I can lean on it when up on a ladder but can see it give if I try to lean with my full weight of 200+ pounds anywhere near the center.
Gelcoat isn't really a paint, at least not like latex or lacquer. It is sprayed into the mold before the
fiberglass is applied to the mold so the two bond together, it has greater thickness than paint and seldom flakes off. It does develop "spider web" cracks with age because over time the gelcoat becomes stiffer than the
fiberglass and the difference in ability to flex creates thin cracks. As can an impact. One can "paint" a new coat of gelcoat but it isn't the same process as regular
painting.
Many have gone ahead and painted old gel coat with good results. With approaches ranging from fine nap roller and brush used to "tip" the runs and lines as you go along to taking it to an auto
painting place and having it done. Probably biggest headache for
painting (aside from removing old paint if it was painted before) is getting all wax and silicon from caulk off the surface. Old gel coat is porous, holds that stuff. Paint over contaminants will not go well.
Some pictures to go with your descriptions would be helpful. Old aluminum
windows are a thing unto themselves.
Doors are also their own subject with a number of things that can need fixing or tweaking and different approaches available to handle the different situations. Also if the door frame attachment at the bottom rots the wall shifts and door won't fit well. Or roof can drop and walls can bow a small amount over time. FG is flexible after all.
Sort of interested in your layout. Mine has a closet at the middle between door and rear dinette/bed and a sink & stove across from that on the street side. Couch/bunks at the front. I know at one time Scamp put AC in the closet, or under sink. Not sure if they did it on the early 13's mine is pretty basic. I know folks added AC there. One thing of concern is the closet and black upper cabinet brace support the roof. The interior objects provide corner bracing both vertically and horizontally. Some of the older ones got gutted or heavily modified without taking that into consideration.
It looks like the black metal is supporting the upper cabinets but it is actually using the upper cabinets to help keep the roof from slumping in the middle by shortening the span of the roof front to back.