I am not sure as I have never done it before but my guess is it will not be nice and flat and shiny like a nice summer coat which is putting me off doing it, if you don't intend to show and his getting hot I would do it as it doesn't really matter then.
Depends entirely on the horse. I clipped mine off last March, and it had no impact on his summer coat at all, but then he doesn't finish shedding until July, and his summer coat was nowhere near in sight yet by March. Even if it had, I'd clip for the horse's comfort. I probably wouldn't clip just to avoid moulting though.