Yeah Im board with the Cora/Queen of hearts theory- the fact it was her father taken & put in a magical safety deposit box wall much like Regina uses, its a bit hard to think it would be anyone else 😉
If all the other fairytale lands were destroyed then the Queen of Hearts needs to be somewhere, and if Cora is the QOH and we’re speculating that Cora preserved that piece of TEF then she’s possibly there now?