With this approach it’s going to look as though users have created topics and replies via email. Staged users can only create posts via email. You’ll get an invalid access error if you attempt to create a non-email post for a staged user via the API.
Something about the staged user approach doesn’t feel quite right to me. It might be worth polling your users to see if they have any concerns. If you could get permission from your users, you could just create active non-staged users on the mirror site, then post their topics and comments via the API.
Passing staged: true creates a staged user. I’m assuming a user created in this way will be able to post via email. My local dev site isn’t configured to send emails to Discourse, so I can’t test that at the moment.