Which currently available treatments is(are) most promising and why?
Corticosteroids (e.g., Dexamethasone)
Hydroxychloriquine w/ or w/o antibiotics
Vitamin D supplements
Blood thinners (Asprin/heparin)
Which would you give to your 75 year old friend who gets COVID tomorrow and there are no doctors around?
Dexamethasone shouldn't be given prophylactically, as it is an immunosuppressant and seems only to be efficacious in those who are on their last leg in hospital.
Heparin would do in my wife, as she is allergic to it. This happened in a hospital a couple decades ago, and the only thing that saved her was a synthetic heparin that had just become available at the time. In short: there are different kinds of heparin, and one kind can be fatal for some.
Low dose aspirin: I take this daily myself, but for other reasons. Nonetheless I'm glad for the (possible) extra defense against clotting. It's possibly the safest thing on your list except for...
Vitamin D: since early April I've gone back to taking a daily multivitamin to help ensure I'm not deficient in vitamin D, seeing as I live in a boreal clime where (as you may recall I said many years ago) a moonsuit is necessary to face the elements during the seemingly endless winters. To get the full benefit of the vitamin D in your body -- to metabolize it -- you need magnesium. The multivitamin also supplies this. So make sure you get your Mg. Nuts like almonds and cashews have a lot, for example. In general just eating a balanced diet is the best defense really.
Remdesivir can have some nasty side effects, but I believe it may be beneficial against Covid-19. Your hypothetical scenario indicates no doctors are about, so would I take it as a treatment, or give it to someone? I'd have to read a lot more about the drug first, and its possible interactions.
Hydroxychloriquine w/ or w/o antibiotics: I think not. It has risks, and any benefits seem hard to replicate in studies.
I don't know enough about most of the rest of the things on your list.