Legal escalation options for Wizz Air / ADR ?

I don’t know the nature of your claim, but that’s a lot of cash to be owed. Particularly if you are not familiar with MCOL and the cost in fees of a large application, my inclination would be to jolt ADR into action by making a formal complaint to them and either simultaneously (or after giving ADR seven days to get your claim moving) writing to the chairman of the CAA to complain about the failure of the arbitration service.

If you issue a claim via MCOL, your ADR claim will cease to be valid, but you could still go to MCOL if the arbitration were unsuccessful.

Unfortunately Wizz has a terrible reputation in this area and there is a risk that even with a successful arbitration (MCOL) they won’t pay up. Theoretically, after MCOL you can take enforcement action but as a practical matter that’s more difficult and costly than some here suggest with fanciful ideas of arresting aircraft. If they didn’t pay after arbitration, the CAA would deal with this quite efficiently.

Well done for the win and persevering down the slower but easier/safer route. Wizz is under a lot of pressure from the regulator to pay unpaid claims and also to reopen all previously rejected claims going back to March 2022. I don’t think the adjudicators at AviationADR or CEDR have the power to award interest.