Your ticket is covered by the Montreal Convention and at 21 days a bag is considered lost, even if later located.

At this point I would make a claim for the lost bag (car seat). You are entitled to fair market value up to a maximum of approx. $1,650.00.

However, due to AB's insolvency, that claim will simply be yet one more unsecured claim extraordinarily unlikely to ever be paid.

I would email AB, asking that it respond with its claim number. Whatever you get back, print out and submit to Chase, noting that because AB is insolvent, you are unable to obtain more.