So, here's my summary. In disputing with the credit card company, I don't have a whole lot of legal basis to win a chargeback, and in the end I might just be wasting my time.
But I would not have done this (paid the collection agency) if I knew it would cost me another $3000 in taxable income on a 1099-C.
So, we'll leave it in the hands of the credit card agencies. If I win I'll just get a larger 1099-C for this year, and have extra money to pay the taxes. If I lose, then life goes on with me having quite a bit more income than expected
.