Yeah, that’s going to be a problem, because each message will be assigned to a new, different user. That’s why Importing / migrating mailing lists (mbox, Listserv, Google Groups, emails, ...) doesn’t allow that by default. You need admin rights on the Google Group to download the full messages including email addresses.
Yes, Importing / migrating mailing lists (mbox, Listserv, Google Groups, emails, ...) works with mbox files as well as each message in it’s own file.