This set of issues is pretty much par for the course for a large import. Whoever it was written for didn’t care (perhaps not enough to notice) about the issues you describe.
Which sounds like a bug in Drupal or the database itself (duplicate ids shouldn’t happen). I would likely have modified the script to test for and/or catch the error when there are duplicates, but your way worked (unless there are more still).
You can look at other import scripts that create post permalinks. The import_id is in the PostCustomField of each post.
It’s either in base.rb or the username suggester. It mostly just works and there isn’t’ much you can do to change it.
You probably don’t want to do that. The issue is that the posts created by those users will be owned by system. You can look at other scripts for examples of how to deactivate them. fluxbb has a suspend_users script, which should help.
fluxbb (which I happen to be working on now) does that. YOu just add something like this to the import user script:
location: user['location'],
Gravatars are handled by discourse core, so the script does nothing to import them; it just works. You can grep the other scripts for “avatar” to find examples of how to do that.
Look for examples. . . . ipboard has import_private_messages.