Hi andrew,
Both are highly breathable so it’s unlikely that either would contribute to sleeping hot but wool would be more temperature regulating (in both directions) than just the cotton (the Dormeir has both wool and a cotton terry fabric). Wool can absorb up to 30% of its weight in moisture inside the fiber itself without feeling damp.
It would have some effect yes. I don’t know how to quantify “significant” so it would probably be more accurate to say “noticeable for most people”. The cotton protector is thinner and more stretchy so it would have less effect on the “feel” of the mattress.
No … a protector is designed to protect the top of a mattress from normal moisture, body secretions, and skin flakes (and accidents of course). For those that have more severe allergy issues and want more complete protection against dust mites then a mattress encasement that completely surrounds the mattress can be a good choice but if it doesn’t have a membrane then it would also need a mattress protector in addition to the encasement to protect the mattress (see post #2 here).
Phoenix