Yes, the OP's BIL was very responsible and spent a lot of time trying to track this down - which is why, in this case, it doesn't sound like it will be a big deal. And yes, Disney did apparently "blow him off." But why isn't it fair to put a hold on the OPs account? There is an unpaid debt. When there is an unpaid debt, Disney puts a hold on the account.
I suspect what happened is that Disney didn't have anything on BIL's account in late June because it had already been transfered over to "member's debt." Had deisel called then as well, they'd have found the charges and taken care of the problem, but that Disney can't look at deisel's account for her/his BIL due to privacy laws. That there isn't any process for Disney to catch these, look up the original occupant of the room, give them a call, and confirm that they can charge the credit card - that once the credit card is closed, any additional charges simply move to becoming a owner liability. Finally, that most front desk CMs don't have a clue how the room charge process works, and that BIL would have had to have been talking to someone in DVC accounting to even find the problem. Not that he should have known that. Just my suspicions.