Even exposed portions of the hull can be expected to not reach equilibrium with surface air. There will still be liquid water splashing around and also conduction from the warmer parts of the hull. Maybe the sail surface could get much colder but I doubt that it's subject to impact hazard under these conditions.

Possible, but that is also the time when the hull is under the least amount of stress.

So maybe they are worried about sitting on the surface of the Arctic in the dead of winter for an extended period of time and then crash diving?