If I Tick 'Save as Test Data' for a Phish, and then Untick it, Does This Apply to All User Data Targeted with the Phish?

It doesn't matter if Users interacted with the Phish when it was set as Test Data.

If it is later unticked as test data, the Phish will appear in those reports again, and all data within it will be just like it was a normal Phish.  The data always stores to the database, and we can simply show/hide the Phish from reporting based on whatever the value is.