Help! we are looking to insulate a family cabin in central NH, built in the 1880s as a summer cabin. My grandparents put an inch of foil backed foam when they had the exterior shingles replaced, and my uncle put fiberglass batts in the ceiling of the main room, then lined the interior with plywood. 

we are hoping to do the ceilings in the bedrooms and the whole house floor for added comfort on hot days and shoulder seasons. And of course, because it is an old family cabin, I am hoping to do it for minimal cost. 

The ceiling:
I have a bunch of R29 rockwool left from a project my Aunt did on her house (almost enough to do the whole ceiling!). There is an old roofline left under the “new” roof, put on in the 80s. a couple of questions:
-what is the best thing to put between the rockwool and the old boards to keep it from drifting down and littering the bedrooms? ie reflectix or something like that? housewrap?
– There is a small corner of the ceiling that I cannot insulate. I have read stuff saying if you leave anything uninsulated all the heat goes out there, but that seems to defy what I know of physics. I should do as much as I can, right?

The Floor:
All the rooms were built at different times and so have different joist spacing. The whole house is on posts on sonotubes, many of which we recently had replaced. Lots of plumbing and wiring runs under the house, of course, which makes spray foam not a great option. 
Do folks have thoughts? One idea (though the back of the house is almost sitting on the ground) is to put sheet foam under and screw it to the joists. Of course we DO have a critter issue (red squirrels, gray squirrels, chipmunks, mice) as the house is unoccupied in the coldest months, and I have read that can give a nice cozy spot. there is not much room to put hardware cloth under and sheet goods feel really expensive as a cover-up. 

Any advice for ceilings and/or floors is welcome!