Why the spoiler tags, that's pretty cool news. Would it explain all of those things though? I mean since when can gods not show their faces, and why would nobody talk about her? Anyway, the wiki only says the Mima decides to be a god, not that she decides to be the Hakurei Shrine God.
Ack, you're right. She just decides to become a god, not the god of the Hakurei shrine. Though she implicitly ends up deciding to become one of the eight million gods instead of whatever type of god Shinki was, if only because she can't really seem to figure out if Shinki's the same type of god.
I spoiler anything stated about endings, just in case. Though honestly I think people are overblowing ZUN's endings. He asked not to show them for people who haven't reached them yet, not to never talk about them or reveal their text.
Anyways, I included "or at least fit in with" cause I realized that's more accurate. For whatever reason, the Hakurei god never shows its face, and no one knows the name of the darn thing. Therefore, if Mima still existed, her never showing her face and no one remembering her name could simply be one and the same with whatever reason the Hakurei god never shows its face.
Alternatively, instead of the Hakurei god explaining Mima, Mima could explain the Hakurei god. Maybe there was no god originally, or maybe Mima was the original god but forgot (as did most others). After she decided to become the Hakurei shrine god ("again", possibly), something went HORRIBLY WRONG with her methods and she ended up accidentally sealing herself in the shrine, now its god but also unable to ever show her face and to continue to be forgotten and unknown. As a god, however, she now cares primarily more about faith than anything, but... she isn't really getting it, hence why Byakuren can sense her anger.
Well, that's just one of several possibilities.
As for why everyone forgot Mima's own name or never mentions her, um... I guess people in Gensokyo have really really bad memory or something (this wouldn't be the only case of THAT, for sure). Though you'd think Marisa would mention or think about her from time to time, at least. (then again, maybe she does and is sad/happy/apathetic that Mima's gone, but tries not to show it so she doesn't show weakness, similar to how Marisa hides how hard she works to keep up with Reimu)