Big project, depending on how much you want to do. I'd more or less forget about the shower parts, unless you can find a damaged Casita. You could think about removing the whole bath area and use it for seating, storage, a bunk, or something. Your project will take a lot of time. I refurbished a 1996 16' LD and it took awhile, and I was recently retired........but it was a complete rig, and almost everything still worked and was in place.
Start your Casita forums and fiberglass trailer forum searches for wall coverings and the like. Others have gutted and refurbished these rigs with a large project like yours, but the parts were still there, usually.
I've used stainless fasteners to replace a lot of rivets. I think the rivet story is an excuse for a cheaper and quicker production technique. I rivet if it's a "blind" fastener with no access to the other side, otherwise S.S. fasteners. I've seen no fiberglass cracking with S.S. fasteners over 8,000 miles of use.
Silicone or 3M 5200 would be good fastener sealants.
Butyl rubber strips are used for window sealants. McMaster-Carr is a great industrial supply house that has everything you need, and alot more you don't need.
Any protection is better than none for winter storage. I built a bow arch shed to store my rig. Not heated, but I was able to work on interior projects in the winter with a small electric heater inside the Casita.