Even with the 'I regenerated myself from the flames after 500 years, but I'm otherwise a totally normal mortal' explanation I remain suspicious.
Like, for one thing, that still doesn't explain the statue of the seven. Because the options are either:
A) the statue changes every few decades with the mortal archons, but this is ruled out by the fact that Kachina feels the need to comment on the statue matching Mavuika. If the statues always match she wouldn't mention it (or for out-of-game lore purposes, she'd explain that the statue always matches the current mortal archon).
B) The statue used to change but stopped 500 years ago when Mavuika put herself in the fire, and since then the tribes have just forgotten that they used to change. The problem here is that the statues are presumably of Celestial make, and if they were changing before then why did they stop?
You could say, 'well obviously Celestia's asleep currently that's why' except they did change statues. Ei is a twin and Nahida's a clone* so out-of-game you can't necessarily tell if they changed Makoto and Rukkhadevata's statues or not, and Zhongli and Venti didn't die, so that's not even an issue needing to be considered. But Fontaine's statues are Furina, not Egeria.
Which means another two things on top of that; the statues weren't put down after the archon war & then untouched, they are meant to match the current archon (not that it would make a difference in this case, because that would just be more proof that there's something else up with Mavuika) and whoever controls the statues was able to make updates after the Cataclysm (so even if they went to sleep immediately after, the statue should be stuck on Mavuika's successor, not her, if the statues were changing for the mortal archons).
(I started typing this before I found out Xbalanque is confirmed to at least historically be the first Pyro Archon, which makes the statue being a woman even more suspicious if that were possible)
Kachina says the people don't actually know how the mortal archons are made the archon and you'd have to actually ask one. At this point I don't think they've even said if the mortals are replaced after death or if they are able to step down from the role. So perhaps there is some variety of immortal entity involved in the process, like how Egeria is still around to some degree as a pond.
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My guess is that Mavuika is in fact an original pyro deity or seelie of some kind who gave Xbalanque the Pyro authority. (Or potentially Xbalanque himself, or the dragon sage**, but they both have the problem that they're male. She could have been reincarnated differently I guess, but that still doesn't explain the statue being a woman.) There's a line by the dragon sage in the codex artifact set saying he was surprised about an envoy, maybe that was Mavuika?
Then during times of high danger she wakes up fully, like Venti, except instead of just sealing herself away or however Venti does it, she's born and lives a human life before passing into dormancy again. A phoenix dying and being reborn, etc.
While it could be nothing, Mavuika is not the one who suggests this is her second life. Chasca does, and Mavuika doesn't respond to it. You don't even get to see her face as Chasca asks. Then the Traveler asks about the plan and Mavuika moves onto that question instead, neither confirming or denying Chasca's. So that could be a red herring for the fact that this might not actually be Mavuika's second life.
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And if we're going by the comic, Vennessa's tribe is said to have forgotten about the Lady of Fire. Maybe it's not that they forgot about their roots in Natlan or the existence of a Pyro Archon, but that everyone forgot about the existence of a Lady of Fire. It's not like people in Teyvat are unaware of the other archons, they are specifically called the Seven, people know about them and who they are. It's people from Natlan who don't leave not the other way around, so even if Natlan has a vague idea about the other archons, the rest of Teyvat should definitely know how the Pyro Archons work.
Even if you assume that Vennessa's tribe were totally detached from the rest of civilization as they forgot about her because of the Wayobs or whatever, they've been living in Mondstadt for a while, they should be familiar with the concept. And based off Vennessa and her family's interactions, the clan did actually continue to hold reverence for the gods, so for them to have totally forgotten about one they were close to (specifically blessed by, even!) is weird.
So maybe its not that Vennessa's tribe just forgot their connection, but that Venti's the only one remembering it. Venti also says Natlan used to perform rites of combat in the Lady of Fire's honor, which is not really the relationship the people of Natlan seem to have with their archon currently. What are they performing rites of combat for now but the Sacred Flame? There are other ways to explain the seeming discrepancy, but assuming the comic isn't just totally non-canon now, this seems like a good possibility.
I mean a dragon that managed to put a human in place of a divine archon, or a deity that gave their power directly to humanity, seems a very likely candidate for the Irminsul treatment. Maybe the Lady of Fire turned herself into the Flame and handed power to the mortals, and the statue was left unchanged, because what else is Celestia going to do? Start changing the statue constantly to match the mortal archons?
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Assuming the family in her picture was Mavuika's actual birth family (and since they show her interacting with their memories in a cutscene where no one else is present, I'm going to assume they were), then I'm suspicious of her mother. Like, her mother looks very much similar to her. And far be it from me to complain about them finally giving some more unique NPC designs, but that's kinda weird in context.
Haumia has the same candlewick curl and hairstyle (if a bit more restrained) as Mavuika. So its not only that Mavuika was perhaps reincarnated to a human family and then SURPRISE she looks different from her parents with flame hair and a similarity to an ancient statue. She was born to a woman who herself looked very similar to the statue of the seven. So maybe the literal-children-of-murata thing isn't totally out of the window, and she's being reborn in her own bloodline.
(Something else interesting to note that could be nothing: Mavuika's mother's name is Haumia, a Maori god of wild edible plants, who it looks like in at least some areas is the brother of Hine-nui-te-po (the goddess Mahuika's sister, and almost certain namesake of Mavuika's sister). But between being a god of plants and not seeing any real overlapping stories, it seems like an odd choice unless I'm missing something. Other than also being a Maori name, but there are other options they could have gone with.
There is however also a Hawaiian goddess of childbirth with a similar sounding name, Haumea, who is the mother of the volcano goddess Pele and said to have occasionally had children with humans to maintain her bloodline within the human race. Could this mean nothing? Probably, but they just made and are still making a huge deal about the meaning of names putting people into roles so maybe...? Or maybe Haumia was just really into foraging.)
If Mavuika is a reincarnation of a Lady of Fire, that also gives her an actual mirror image. Other than Zhongli, all the archons have an independent mirror image. Even Focalors and Furina despite being parts of the same original person, act independently of each other. Exclusively based on confirmed in-game info on her, Mavuika is a single person who was able to jump herself 500 years in the future. If just being later in the timeline makes you a twin of your previous self every character whose had a flashback has a twin, so no. Mavuika does not currently have a double.
*as an aside, the Dendro Archon replacing herself the same way you clone a plant is pretty clever & I've never seen anyone actually address that.
**I'm also suspicious of the dragon sage. More of the books got added to the wiki over the course of me writing this post, one of which says (to paraphrase) 'the dragon robbed of fire absolutely wasn't the dragon king, that would be absurd everyone knows Xbalanque beat him also everything in this book is 100% in-game fictional what are you talking about'. Which very much makes the whole book seem like a case of they doth protest too much frankly. Especially since that's not even the only incident in the book where something elsewhere attributed to a different person is instead given to the dragon sage, which smacks of historical editing one way or the other.
More insanely, the same story has the dragon sage specifically refer to the Newark Holy Stones, a set of falsified historical artifacts from the real world, the weird context of which presents potential plot implications in multiple different directions for Genshin. But more immediately they mean that the dragon sage knows about the existence of real world Ohio. And it somehow factored into his plan to send the people of Natlan to the moon, or something.