The fact remains that whatever people think it ought to be, the rule is simple. It is AV which is solely responsible to OP if OP is owed anything at all (OP does not make clear that it is that he expects as he got his undamaged bag and does not suggest that he had any expenses).

If AV believes that DL is responsible, AV may settle up with DL. That is just how it works.

AV refused to take a report on a flight departing the US. OP may file a complaint with DOT about that, but I don't suspect that DOT will care much as OP got his bag. OP may pursue AV elsewhere too, but again, I don't see any government regulator caring much.

But, for the benefit of others, if your bag does not show up, no matter where you think it is or who may have handled it, your complaint is to the last delivering carrier.