yes. if you don’t do your own cat mapping in the settings file, new cats will be created as needed during the import. if there is an existing cat in discourse already matching a cat name from phpbb, the cats will be merged.
oh, yes. here is a thread detailing what i’ve encountered. i have some tips and also a repo with my combined updates to the importer.
i can’t remember the exact conditions, but yes, users get merged automatically during the import.