Hello Rutger,
1) AppCloud users are users that are registred with Mendix. With your user base I would stick to local users which are only known by your app.
2) Depends on how you want to do the registration proces. I would simply import your current user base (Excel import it). But you could skip it and only create a user account for the people who want to register.
3) I would stick with the local users. So creating a new account is a local user.
Good luck with building.
Regards,
Ronald