1) Sure, you can refinish the sub-floor. Just be sure of the type of wood it is, and use a high-quality primer to seal the wood (and prevent bleeding.) When you buy a finish paint, use a high-quality acrylic latex paing. DO realize that you're walking on a painted floor. If you think your painted walls take a beating, wait until you see what happens to your floor when you walk on it! (In short, I don't recommend this type of finish for a floor ...)
2) The wood has developed gaps over the years as a result of expansion and contraction. By 'filling' in the gaps, you'll prevent the E and C from occurring on its own and, thus, whatever you use as a filler will begin either popping up from the gaps or cause the floor to buckle (because it can't expand and contract.)
You didn't say what you use to have for a finished floor. If it was carpeting, I'd suggest you stay w/it. It sounds like the floor 'moves' a lot. My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America!