QlikView treats users defined locally on the server in the same way as users defined in an Active Directory. The users are defined as Servername\User instead of Domain\User.
So your answers are:
1) Yes, but the user will always be prompted for their username and password unless the username and password of computer are the same or you same your user credentials.
2) Yes, in the security tab of the file you define which local users have access.
4) How you define your permissions does not limit your ability to migrate to an enterprise server.