1, yes, but so complicated that it really isn't worth doing. Your better off just loading a new hosted mod. Tell your players to take a quick break, get a pop, empty the bedpan, while you load a new mod.
2, use NWNserver, (in your nwn file) to host the mods. when the player logs in, they will have their characters. The characters will be in your server vault file, under their login names.
3. depending on the machine, 300+ seems to be stable for most folks. Some have made over 700 areas, and have stable mods with multiple players. I have run mods as high as 600 areas , for my sons and me, and not had a problem.
Good luck!