Initially I thought that you were trying to open that onion link in a standard clearnet browser, but you are right. It won't load over Tor.
for those two situations, it would be better if you recalculated the exchange amount instead of issuing a refund.
True. I see no reason to refund the money unless the customers want a refund. The exchange could offer its clients both options and allow them to choose between getting a full refund or swapping a smaller amount.