This is not hard, especially with modern finishes. Assuming that you are not going for a Hinckley-museum-piece-of-art, I propose a middle of the road approach. Some people swear by traditional finishes but in my opinion, modern urethanes are virtually indestructible and it is very difficult to mess up the application. It takes at least 3 coats for a finish to start to look good. Inside applications should have about 5 coats for good coverage. Outside, I use 3 coats Cetol Natural finish. I have a lot of experience with interior and exterior finishing, so I'm somewhat biased
1-Replacing dingy cabin sole
Sand the ply with 220 grit and finish with Minwax Gloss Spar Urethane. Seal edges/bottom with epoxy. My table and cabin sole: VICTORIA (and her mistress)
2-Replacing a crumbling plywood mount
I would not use ply in this application. I would use Marine plastic lumber or solid teak wood finished in Cetol Natural. If you do use teak, do not buy retail from WM, it costs a bundle. Go to eBay or an online lumber supplier (M.L. Condon, etc) and mill it yourself.
3-The thin plywood
Sand with 100, 150, 220 grit and finish with non-gloss Spar Urethane.
4-Teak toe rail, cockpit rail and couple of grab rails
Scrape, sand with 100, 150, 220 grit, then 3 coats Cetol Natural
Some of our exterior teak:
VICTORIA (and her mistress)