DISCLAIMER: THIS POST DOES NOT CONSTITUTE LEGAL ADVICE, BOTH NOW AND IN THE FUTURE. IT IS SIMPLY ADVICE GIVEN IN GOODWILL AND WITHOUT MALICIOUS INTENT TOWARDS THE ORIGINAL POSTER NOR TOWARDS WIZARDS OF THE COAST.

I'm no legal expert, but the Terms and Conditions appear to have verbiage that would prevent you from suing over this issue, namely, General Terms sections 7, 9, and 10 (which are part of the MTGA EULA.) However in section 20 it does state that you can initiate arbitration (through writing) in an attempt to rectify the situation and that this arbitration can include monetary reconciliation. That out of the way, my advice is to just keep asking for a refund on each individual event as that is what WotC is most likely to do. In the event that you do attempt arbitration, WotC can hide behind already refunding you some or all of the gems as evidence that reconciliation/compensation for this issue has already been completed/given.