If there is a login ui, why wouldn’t it just write out to $HOME from the perspective of the snap, ie ~/snap/snap-store/current/.snap/auth.json?
Interesting, then it might be even nicer: “if you want to get notified of firmware updates, connect this manually” 
Not auto-connecting fwupd would be a regression from today’s gnome-software in Ubuntu. And without connecting it we can’t check for updates.
Yes, it will just use $HOME for reading/writing. So if it has a different $HOME to the snap command line then it will have a separate credential. For the best user experience you’d only have to login once from either the command line or the GUI. What we can’t do is have read access to the command line credential but not write access - we need read/write or use separate credentials.
I think separate credentials will be fine
I also want to add my unofficial endorsement for auto-connecting fwupd. Given the meltown and spectre-class vulnerabilities, users should not have to opt-in to firmware updates. I even use this as an advantage of Ubuntu; “you get automatic firmware updates without doing anything”.
I also disagree firmware updates are “not frequently done”. If your computer is well supported (like most dell laptops), you get very frequent firmware updates. I’m guessing I got ~5 firmware updates in 2019 alone.