You've already outlined your case on the official forums, so if the GMs were so inclined they could take action against you right now for admitting to account sharing.
The fact that someone else is impersonating your character is also worthy of the ban hammer, but given that your acct will likely be suspended at the minimum (if not outright terminated), the only real recourse I can suggest is have someone else whom you trust in game contact Square Enix and lay bare the details of the impersonation.

Just be prepared to lose your acct altogether. Regardless of either outcome you will not know the results of any action SE may take against the other party.