regit Reporting back that I was ultimately successful in getting reimbursed by Google!
What you said lined up. They told me to provide proof of purchase and login again to my Google account, which at that point, made me realize it was probably something GrapheneOS related blocking their ability to see my device details on my account.
So, I made the decision to revert to stock and I logged in. They were fortunately reasonably quick and responded back 2 days after I sent them a pdf of my order details and confirmed that "I logged back in." I didn't mention I was running an alternate OS since I didn't want to risk anything and I prefer my privacy.
(I tried to limit how much I used stock pixelos and essentially went phone less for a few days). Once I got the email saying they approved me and I received the payment, I rushed back to GrapheneOS lol.
I will say that if someone is in this position, just be persistent and don't take no for an answer as I see some people unfortunately did. No need to be rude with the customer service, just be persistent and they'll escalate it.
Thank you so much for the outline of what to do. It was very helpful.