Honestly - I would start with the Dept. of Ed directly. They have ombudsmen that can help you navigate issues with servicers
I don't know if they can help with your issue, but it sounds like something that is right in their wheelhouse.
ETA: that website is a hot mess. Here, have a toll free number 877-557-2575
Oh, dang - I'm sorry that they don't help. It's been an age since I contacted an Ombudsman and they used to be more useful.
So yeah, a letter with all of your proof to Navient, CMRR (certified mail return receipt) for a record, and then maybe either a lawyer's help or filing a complaint with the CFPB if nothing changes. It sucks that you're going through this, and I know servicers are slow on updating/fixing things on the best of days