Until we learn what happened at the shrine I'm going to sarcastically chalk it up to gremlins.
What Maribel saw at the shrine may or may not be entirely unrelated to her becoming Yukari. I wouldn't bother looking too much into it because there's nothing either way that says that whatever she saw at the shrine had a hand in her becoming Yukari. Maybe what she saw at the shrine DID lead to her becoming Yukari. Or maybe what she saw at the shrine was something else entirely and was no factor in that. We don't know either way and there isn't much we can do to pursue that matter further (far as I can tell), nor is it necessary to her becoming Yukari anyways, so there's no need to bother looking into it, I think.
While it'd be nice to know more specifics about Maribel's life events, most of that is more a luxury than anything from what I can tell, at least concerning the "Maribel changed her name to Yukari" (or became Yukari, or however one wants to word it) theory. We have more than enough "facts" (well, the closest thing we can get to facts considering that the related official stories are usually told from a character's perspective) to likely discern it, already.
Maribel Hearn's relationship to Yukari Yakumo is "Well, there was a man named Lafcadio Hearn..."
Lafcadio Hearn changed his name to Koizumi Yakumo.
Yukari Yakumo is NOT Yukari's original name but instead something Yukari chose, herself (implied by PMiSS' wording and "probably" in CoLA according to Rinnosue. Again, not 100% fact but it's the closest thing we'll ever get to a fact barring ZUN's word himself)
Maribel's power is evolving from the ability to see boundaries to the ability to manipulate them (CoSD according to Renko, and again in her latest stories where Renko says Maribel's power has evolved to the point where she is more like a youkai than a human. Again, this is Renko's opinion but it's the closest thing we'll ever get to a fact barring ZUN's word, himself)
The only missing fact is "Did Maribel travel back 1,200+ years to the past?". It's the only "event" that's needed for Maribel to become Yukari at this point. Which is why I called it the only missing puzzle piece. Again, even this event is at least stated to be possible (CiLR, where it's stated people sometimes get spirited away through time itself), and Maribel has already travelled back to the past before (her memo discovered in PMiSS). All other events like what she saw at the Hakurei Shrine aren't really necessary to that, even if they'd be nice to know.
