Good luck with the bags, that does really suck -- I suspect one of their future process improvements when a lot of flights are in danger of being canceled is what I read they did later on -- they were checking bags but having people hold on to them until close to boarding time (I don't recall the rest of how it worked), specifically to avoid this.

What did you end up doing about the car seat -- did you purchase a new one? I'd send them that receipt and ask for payment, and push back on any suggestions about reduced value. While there are other variables, a car seat typically lasts the use of the child, so having a newer one has no additional value to me. Tell them if they find the old one and really they're welcome to exchange it if they really believe the new one is more valuable. But my impression is that they're currently prepared to be a lot more generous than usual on luggage and alternate travel reimbursements, so include any other additional expenses from not having your bags. I know airlines often claim you can't possibly have expenses for such "at home", which is true for some things but not others. I certainly have plenty of underwear, socks, T-shirts at home, but if my only suit is in the bag and I need it the next weekend, then I'd still need to replace it.