I'm obviously concerned about my catalytic converter at this stage

As well you should be.

The odds are near-certain that you (or rather, the morons who overfilled your crankcase) have killed your catalytic converter.

I hope you have bullet-proof documentation from the VW dealer who fixed the problem (as best they could) in Cincinatti.

Bring that documentation to the miscreants and insist that (a) they pay for the Cincinatti repairs; and (b) they replace your catalytic converter.

When they inevitably refuse, don't argue with them. Just mention the three magic words, "Small Claims Court", and then do it.

Good luck...

tanstaafl.