Apparently this occured on a work-related trip and your employer's instructions are to decline all insurance. This presumably means that your employer either self insures or has coverage for this. Simply turn the claim over to whoever that is in your enterprise and have them deal with it.
No idea of the size of your outfit, but some places handle this in house and simply negotiate it out and others carry their own master policy and turn it over. Presumably you write up a report noting that this was not apparent at return as acknowledged by both you and National and that it is the sort of thing which would have been noticed. Forget all the stuff about what you would have seen over the 900 miles, this is about the "clean bill of health" at return. Not as though we're talking about damage to the undercarriage or whatever.
Enterprise-level people get more attention. Yes, there is the risk that if it is not dealt with that it will hit your credit card. But. stay on top of your employer's contact.
Also, check your own insurance. It may well have a deadline for notice. If so, provide that notice immediately,. Ultimately, that protects you.