I would hire a local lawyer and have them write a "demand" letter to the state agency that administers the retiree benefits. Yes, you'll probably have to pay $1k or so to the lawyer, but it will be worth it to get the benefits.

Companies/governments tend to take letters from lawyers threatening to sue them more seriously than a front line call center employee answering a call from Jon Doe.

You could also file a complaint with the state AG office - not that this will necessarily get anything resolved, but it might just get your case file in front of someone who actually cares.

PS - the type of lawyer you want to search for is probably someone who deals with "administrative law" / deals with government agencies and such. I did a quick Google search and came across this firm. Not say you should use them, but I think this describes the type of firm I would hire.....

https://www.helmerlegal.com/practices/administrative-law/

https://www.lyonspc.com/administrative-law/