Sorry to hear you got an unpleasant surprise. I'd also be quite disappointed. But I'm afraid that if escalating, you would be trying to argue on a "he/she said to me three months ago" basis. That's not among the criteria for qualifying for Mosaic; it's having enough segments/base points, and B6 would be in a quandary trying to deal with others who came not-close-enough and wanted to put forth that argument.

It's easy to keep track of what has actually posted to the account, and after the November flight (at the latest) I would have done the math and adjusted my December flight accordingly. (Of course, the fact that B6 is rather slow at posting completed flights doesn't help.) Or done a mileage run. If you make such substantial use of the Mosaic benefits, it might be financially worth your while to book a high-priced run during the current double-miles promo period, but work the math out on paper, not in your head, and certainly not in the head of anyone else on the telephone. Sadly you now know that there's too much risk of error there.