Repeating my question from up this thread - when UA rebooked you, is the rebooked reservation in the same fare class as originally or different? If you don't know the fare class of your original return non-stop, it is probably the same as your unchanged outbound.

If UA booked it in the same class of service, I have nothing to add other than pointing out to TK that UA already secured the FF inventory which is why the booking is in the FF class (again, most likely X.)

If it is not booked in the proper FF category, then the conversation with UA would be that TK can't ticket unless the rebooked ticket is changed by UA to X (original class of service.)