Agreed 100% on the account proliferation and type asymmetry points. The way things stand, right now, the user's choice of account provider will determine what actions they can take on the fediverse as a whole. It is a wholly unfortunate state of things.
An interesting exception would be Owncast's "Fediverse auth" option for stream chatting. That sends a One-Time code to your mastodon inbox for authentication.
As @jackalope@lemmy.ml suggested, Solid would be a shoo-in for your "User Data" server. If, that is, Solid could shake off some of its sheer conceptual gravity. People say the fediverse has a geek problem, i.e. only geeks use it. Well, I think Solid has a worse version of that problem. It is only approachable by the deepest loremasters of geekdom. They are also still vague on its actual operation. What's more, they are still deliberating what their actual security model will look like.
Which makes me sad, because the Solid sounds exactly like what we architecturally need.
EDIT (3:25 am EDT): Just wanted to add on here, I really think that "linked data" and SPARQL were bad, possibly self-defeating decisions for the Solid project. I sorta see their motivation–they want that sweet, sweet flexibility. But I think this approach is not a good solution.
EDIT again: added links