As I know basically an oauth account with the login-oauth plugin is independent from the normal(?) account.
However since I wanted to let normal account owners log-in with the oauth method provided by facebook, I have modified small parts of the login-oauth plugin and it works well.
Do you think that this feature is valuable for the main-stream?
I’m certain the team would welcome pull requests. That’s the ideal avenue to have this discussion because people can also look at the code.
Yes, I would do it. I just faithfully followed an instruction like a good kid.
“Please ask first in Gitter or in the Forum before embarking on any significant pull request (e.g. implementing features, refactoring code…”
Even though, this is not “significant” but still “implementing feature” 
I think they just don’t want people wasting time preparing huge pull requests that will most likely be declined. For big things, it’s best to make sure the team is even open to it. For minor feature improvements, go for it.