This won't qualify as IDB under US rules as the flight was not oversold.

Expect both airlines both airlines to blame each other so I would file a DOT complaint against both, so you get written responses from both airlines.

Looks like AC is most likely to blame. I would send in a complaint to AC and a request for reimbursement of the extra money you had to pay failing which you will take them to court.

I would also do a chargeback against the original AC ticket for a partial amount ( ORD-AUS) for services not received, which is the amount you paid for the extra tickets.

I don't why the law allows airlines to dodge IDB compensation when they are denied boarding despite having paid for their tickets. If the airline responsible for this denied boarding to to pay compensation and a massive fine the would be a lot less of these situations.