I think I’m also facing a similar kind of issue with my migration from gitlab.com to self-hosted GitLab server
Since you have mentioned “Import from gitlab.com button will not work on self-hosted. i have tested the same from my end.
- i created a backup using the export option in my gitlab.com project
- then i go to my Self-Hosted GitLab server and used the “import project” option to import the downloaded tat.gz export file
- in the next window (Self-Hosted GitLab server) select GitLab Export
- Once everything is completed we see this behavior the user name in gitlab.com changes to Administrator in self-hosted GitLab after the migration (The user name will be replaced with the user who imports the project in the destination server, in my case I’m importing the project as Administrator)
I have noticed that you mentioned Congregate in your Reddit post . Did you get any chance to test how the internal GitLab migration tool works (Congregate). i tried a little bit but nothing happened.
Hope you have found a proper solution for the issue mentioned here, If yes please share 
I will also follow up if I find any proper solution for the issue that I’m facing during migration.