Hi SKahlam,
As far as I know, "apply the same ExchangeGUID to the test account" this step applies to the migrated mailbox which was created by creating remote mailbox, if the mailbox which has been migrated back is an original on-premises created mailbox, you do not need to apply the ExchangeGUID.
Additionally, as the error displays "Unable to update this object because the following attributes associated with this object have values that may already be associated with another object in your local directory services", it indicates this test mailbox has some arrtibutes (or proxy addresses) that already exists on local AD. And does this issue occur after importing the PST file or before it?
In this situation, if possible, I'd like to suggest you migrate another mailbox to Office 365 and move back, check to see if you can reproduce this issue. This step will help us to confirm whether this is a specific mailbox issue.
Your time and efforts on this issue will be pretty appreciated.
Best Regards,
Bourne Zhang