You don't put a path, Hammer looks in moddir/gfx/env for the skies anyway. Just put "night".
You should not call your pak anything other than "pak0.pak" (if it's a standalone mod), or the next highest free number for an existing mod (i.e. if you already have pak0.pak and pak1.pak, call your new one pak2.pak).
No, he's right, it all moves. Obviously there's something that doesn't on the outside, but all the cliffs do to give your Osprey the illusion of movement.