Hi
1. The scew-in mechanism is normally required to guarantee water protection. I quote Longines: "Like the screw-down case back, it is also how the crown is constructed that plays an essential role in guaranteeing water-resistance. To facilitate setting and (for mechanical watches) winding operations, the case must incorporate an opening. This opening, however, is also a potential entry point for water or dust. To eliminate this weak point, the winding stem is inserted into a tube which, in turn, is inserted into the case. The crown then screws into the thread of the tube and gaskets are fitted to further enhance water-resistance."
2. Glued on. Can be taken of by heating the crystal which makes the glue soft.
Kind regards
Val.