Default [Q] Unlocking serial unknown tablet

Is it possible to unlock an "unknown serial" tablet?
I haven't used the TF201 in a while and decided to just unlock and get a custom ROM. Found that I couldn't (I've been serial unknown since day 1 and never bothered to RMA). Anyway, I discovered Ratchet and set my serial number but that hasn't helped. Left the tablet running for a couple of days on WiFi and OTA updates and the unlock tool both fail.

So right now I have it on 9.4.2.15 with root (since I don't think it's possible to get root on any later fw).

Do I have any options?
Will Asus RMA it free out of warranty?
I don't want to spend any money on this and I'd prefer not to lose root, but if I can't unlock, I guess I might just manually update to the latest version. Is there a way to downgrade later (again without root or unlock)?