wrote:

snip

whether this can be fixed by sanding or
something short of totally ripping out and replacing.

Sanding flat is probably a waste of time and money. The flooring
irself should have been either quarter sawed boards to resist the
warping or a moisture resistant laminate. Given the choice for a
second home or cottage, I would go the laminate route. But if real
wood must be put down (people have their reasons), an old timer's
trick is to put some sort of finish on the bottoms of the boards to
slow down the effects of humidity and temperature changes.
As the demand for wood flooring has increased the last few years,
seems to me some of the material in the marketplace has gotten rather
sorry compared to what I saw years ago. Good luck on your choices.

Joe