You don't sanitise the wool covers in the sun - you have to dry woollens in the shade (on a flat surface) You do only have to wash them once they get stinky though, which is probably around once a month here? unless they get pooped on.

Are you going to be using flats at night as well ? I would be worried about the chance of compression leaks with the combo of wool covers and flats - just because flats arent' as absorbent, as say, bamboo or something like that. It really is about finding a system that works for you though