wrote:

With more than a month to go before the start of the trip,
United Air and the tour operator DID negotiate it;
instantly, by phone. ...

I don't know that Air France won't negotiate; I just know
that its institutional structure is making it impossible
to find out.

What's to negotiate? You probably bought a ticket on United that allowed a
credit if they were notified ahead of time. You bought a ticket on Air
France that didn't allow any refund or credit as part of the contract
terms. There is nothing to negotiate, based only on the terms of the
contract. Such contract terms are common on a number of air carriers.

If Air France eventually allows a credit, it would be because of good will,
not the terms of the contract you agreed to in purchasing the tickets.