Update: I was able to fix the problem and regain access to my character and progress. Here’s how.
I had a conversation with #Bethesda Support about unlinking my Xbox Live account and ended the conversation with telling them directly not to do it because they told me it would result in loss of characters/progress/etc. in #Fallout76.
I asked them to unlink my Steam account from a secondary Bethesda account instead so I could transfer that over. (The ultimate point being that I would start playing Fallout 76 through Steam instead of Windows Store).
Can you guess which account they unlinked anyway?
Almost 150 hours of time invested. $130 of Fallout 1st, extra atoms, character, items, progression, all gone. Just like that.
If they don’t restore it, I guess I’m done playing Fallout 76. I’m not reinvesting all of that time. I don’t have that kind of time to waste redoing the same things.