That is simply what is referred to in legal circles as BS.

The requirement that you have a ticket, reservation and have timely checked in seems clear from the language of the Regulation and nobody disputes that. The requirement of a boarding pass, especially for a flight which the carrier cancelled is a red herring.

Given that you will need to pursue this in a French, or at least a court in the EU and that you liven in the US, I would turn this over to a claims agency. It will cost you 25-33% of the claim, but the fact is that it is different than $0.

You might try one more note stating that they are simply wrong and that you will turn the matter over for action in French courts and that you will also complain to DOT here, asking that DOT evaluate the carrier's landing rights. DOT won't act based on your complaint, but you can't be the first.

If you get pushback again, let the claims people deal with it and move on. You are throwing your valuable time after people you can't effectively chase.