Mine 76 Argosy 26 is same year as yours, maybe similar interior.
1. Yep, hard to find curtains when need. My suggestion is to look at an auto or vessel trim shop or upholstery shop. I got, actually begged, a local Upholstrey shop to do, and did the interior cushions too. Used original drapes as a guide. Bought a heavy fabric. Great job too. I bought new plastic hanger glides with stretch fabric, you can find online. Guy sewed on nicely. Sun did brutalized stretchy fabric which disintegrated within 3 years. So looking to replace. Curtains still good. I did find online curtains, but my upholstery guy’s job was much cheaper. Unfortunately he has moved somewhere out west.
2. I saw vinyl folding door online once, but decided to remove mine an use a school UL Cards flag.
3. I got a plastic shower curtain and customized by cutting several inches off width and length which works well until I find better solution. Be Safe.
Be Safe. Mitch