1.- yes, you can roll all of your characters
2.- yes, npc´s will still be there
3.- changes: not really too many, the game might be slower when pausing, and the buy/rest screens (like in shops or taverns) change a bit. Best idea is to save the game, then move the saved game folder from the MPSave directory to the save directory, that way you can load as if it was a single game, and pausing will be as fast as usual, and the screens will remain the same.
4.- Select Multiplayer, then in the connection type choose "serial", then create game. Roll your characters, mark the symbols besides them and start the game. Then id recommend to save it and move the folder from "mpsave" to "save" directory, under your BG folder.
If you need further help, dont hesitate asking here !
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"Sigh" -Ertai, Captive of the Blinding Angel