Legend of the Flower Sacrifice in the Mountains🌸 (Most plausible theory)

I've settled on the most likely Kunistu-Gami theory. It solves my anguish on why Soh's face is different and why the new goddess has Soh's face but Yoshiro's accessories. Why does Cursed Soh resemble both of them?
Why is Soh so calm and composed? And all these questions I had when I kept pushing my 'they're one' theory.

To preface, I really like my old theories and the theories of others. Looking at the game from the perspective of Yoshiro and Soh being one is enjoyable. All theories excite me but the is the most likely one.

I figured out this particular theory after using Google to translate the kanji on the scrolls as well as the Japanese text in-game. Things get lost in translation. AI isn't always accurate with translations so I'll post them up and simply use them as a guideline alongside individually looking at kanji. I can't read kanji but I did what I could.

She wears orange so this is cute but this isn't correct

The most likely one

This is the Chinese translation. It's true lore-wise but not what we need right now

All these pop up because of the kanji also being Chinese. I've also gotten 'Apricot ritual' and 'Apricot Flower ceremony'.
One with 'mountain' in it is going to be correct because it's specified the word being used is mountain. 

I show all these translation, even if they are incorrect, because the apricot blossom=Yoshiro in orange.(funny coincidence)
Flowers= both Soh and Yoshiro are covered in them. It's feminine design choice but also is a subtle nod to the kanji similarities? Yoshiro is also compared to a flower in her lore plaque. The villagers wear flower masks by default. Everything is part of 'the flower' ritual. I just thought it was cute but these aren't accurate...

大山祇世代祭祀傳

It's the story of the 'mountain generational ritual sacrifice', Yoshiro's kanji is in there.

Yoshiro is the name given to the sacrificed maiden. She's nameless in a way, just like Soh is.
Yoshiro and Soh are titles bestowed upon them in this world.
In the play, they're simply called the Maiden and the Messenger.

'Human Sacrifice'

Here, the Maiden is offered to the Goddess as a human sacrifice. She becomes a substitute for the Goddess and is given powers to purify the defilement. (Nanamagari represents this ritual)
The skeletons are there to signify this is a sacrifice of her life.
(When Spirit Stones are replaced, do they revert back to their human bodies and have to be buried? Or do they actually physically liquefy and get absorbed?)

This is showing her path and future. First, she is a Maiden, then she becomes the Messenger. I don't believe this image is meant to be taken as these literally exist side by side but rather as a story-telling visual to show the path she walks. (I choose to believe this because Soh's armor is not physical, it's mystical. There's no reason for these clothes to exist. The villagers can't even see Soh.) 

Kunitsukami

When the new Maiden is chosen, the new Yoshiro, the previous Maiden returns as the Messenger. To protect and guide her along the path she once walked herself. The scroll's name is curious though...

More on names:

I call Yoshiro's name a title because of what the kanji means.

So, she's this generation's substitute for the Goddess, hence 'walking the path of the Goddess' and why she represents the gods to humanity and humanity to the gods. The villagers bring her offerings and treat her with great respect because she's the acting Goddess.

There are these Shinto terms which use a kanji from her name:

Credit goes to an article on Shinto shrines. Link 

Yep. This generation's substitute for the Goddess.

Her title in older ads for the game was 'Spirit Stone Maiden'. She's seen as an object housing a deity, not as a person.
(Kunitsukami as the name for the scroll of Soh...Soh is in a way a 'kami' but one made on earth. The Messenger was once a Maiden acting as a deity. When she returns as a spirit with the power to stand against the defilement alone, she's still acting as a deity, hence Kunitsukami? It's why Yoshiro treats her with great respect but also because she's the former Maiden.)

Let's look at Soh's kanji:

Ancestor because Soh is a former Maiden, as Yoshiro is the current. Soh is the one before her. Esteemed and respected, naturally because she's now a messenger of the deity and fulfilled her duty previously as a Maiden. A senpai, perhaps? (ssshhh)

(Extra note: Yoshiro is said to guide villagers along their paths and Soh bestows power to them through the masks. Soh does both of these things during the game, because she's a Maiden who's now a Messenger.)

Soh is the previous Maiden.

In the first ending, Yoshiro approaches Soh, sees Soh's face and is left with the mask while Soh passes on. We're given a look at the Spirit Stone. Then we see a new Maiden and Soh watching over her. The new Soh is our Yoshiro.

Only the prettiest get to be Maiden :x

This is confirmed in this scene here:

The Cursed Soh we fight is made of the previous Spirit Stone. Soh's physical body. (her crystallized pose seems a little different to our Yoshiro's pose.) Spirit Stones are the bodies of the Maidens fully corrupted by defilement. The shrine is the tomb of every Maiden who walks the Path.

Cursed Soh looks like Soh and Yoshiro because Soh was once a Maiden. All 'Sohs' are Maidens first. Many girls have been sacrificed this way as hinted in Cursed Soh's plaque. (Possibly 7 Maidens in total, by Nanamagari's name and everything being unlocked on the 8th cycle as Yoshiro is the 8th. Lucky 8.)

Cursed Soh's lore plaque

I thought it was countless girls but it only reflects as such. The reference to a mirror is because we move onto the next cycle using a mirror.

They've been sacrificing girls for a long time and these Yoshiros absorb all this defilement. Again and again. 

This defilement appears to accumulate.

When we defeat Nanamagari, it's revealed to be holding Soh's Spirit Stone(now as liquid defilement) in its belly, as a womb would hold a child.(Because it symbolically 'creates' the Maiden/Messenger through the Sacrifice. This creature resembles the man in the Human Sacrifice scroll who in the drawing has a faint circle on his body right where Nanamagari holds the Spirit Stone.)

Yoshiro absorbs this mass of defilement once we defeat Nanamagari, and becomes the Spirit Stone. This occurs over and over and over and all that defilement keeps piling up. Then we get to NG+.

Once Nanagami is defeated, the defilement pours out of it and this time manifests as the shadow Soh and then as Cursed Soh.

Yoshiro silhouette

But why this time? Why now? I suspect it's because Soh is fond of Yoshiro. We know she makes the Maiden laugh and seems to offer her words of comfort or advice maybe? during moments in the tent.

This gesture only happens after she becomes crippled

Yoshiro's reactions to Soh hints at how Soh behaves towards her. Don't we encourage her to eat? It's why she always bows her head at us after eating. 

In battle, if you move close to Yoshiro, she'll stop dancing and stand there while vocalizing. We hear her vocals in battle if we stay close to her. But why? She loves to hum but does Soh like it as well? It's one of her gestures in the tent.

When standing outside the tent, she often looks around. For Soh, perhaps?

Looking around...

How Yoshiro speaks to Soh as well. 'Take care', 'Good morning', 'Thank you for everything', 'Be careful'...

(Yeah, call me sappy but it's all about love. Whenever Yoshiro laughs from something Soh says, it's two maidens laughing together, isn't it? Girls destined to die at young ages, with Soh having already done so.)

Maybe Soh didn't want this for Yoshiro. 

When we defeat Cursed Soh and purify it, this purifying white light comes from it. (What is this light?) Soh absorbs it and is then able to completely purify Yoshiro. 

Soh says something to her but I'm not sure what it is. Dokenai Unai Masa...? It's the same line in English and Japanese.

Soh then ascends to being a Goddess.

This is why she has Soh's face and Yoshiro's accessories. Because Soh was once a Yoshiro and this is Soh. (Why are her ears so big? Does it symbolize something?)

The kanji here is: 御阿礼

I tried matching the furigana(?) and got 'Miarei'. 'Mi' for 'spirit or god'. The word used for gods/the divine. Mikoshi, Miko, etc. People who translate kanji have my respect. I'm trying but I don't know anything.

In the image, they're definitely thanking her and venerating her. But why is it translated as 'Birth of a Goddess'? I don't know. But I believe it's related to the 'Kunitsukami' scroll, in that Soh is a Goddess originating from earth. 

The final scroll in the story is Eternal Pool of Sunlight.

The symbol of Purity and the symbol of Impurity, sitting side by side, peacefully.
Our vices must co-exist with our virtues. The sun always rises and sets, every day. The balance.
There's also the idea that once you reach a certain level of enlightenment, even 'evil' becomes your friend. The way some Asura chose to use their love of violence to protect Buddha. (A theme in Sekiro, the brother to this game IMO) This idea of Asura is also about if you can't let go of your vices, channel them into something positive.

The idea of learning to live with your flaws in a way that they do not overtake you or become your enemy, about wanting the best for the next generation. To break harmful cycles, we have to care about those who come after us.

 
I don't believe Soh had the conscious intention of saving Yoshiro, I believe it was a seed that bloomed within her through her time with Yoshiro and made it so that Cursed Soh emerged.

Why does Cursed Soh point at Yoshiro, then go to the shrine, showing her inevitable fate? There we fight Cursed Soh, who wants to destroy everything and end the cycle this way. It results in killing Yoshiro, too. Anger and Hatred...

Maybe this is why when Yoshiro messes with Cursed Soh, Soh seems surprised but not angry.
As in '?' because Cursed Soh wants to end the cycle and can't understand Yoshiro standing in the way. Because Soh is doing it out of love for her but also hatred of everything else. Maybe even resentment for Yoshiro as well for just accepting this fate?
We can only speculate Soh's thoughts and feelings but there must be a catalyst for this ending and it being Soh's fondness for Yoshiro fits due to their interactions. Or maybe it's about the number 8?

Remember, the Maiden is chosen when she's a child and the Messenger then watches over her until it's time to walk the Path. Once it's time to perform the ceremony, the Messenger is consistently with her.

Every time Cursed Soh goes to kill Yoshiro in my playthroughs, she uses Taion's Lightning. An instant kill, because she doesn't want Yoshiro to suffer.

Let's briefly go back to the cycle again...

NG+ is a rerun. It's the same Yoshiro in NG. Did Soh rewind time to redo it? She canonically has this ability. Or is the implication we're playing as our Yoshiro and that's why we don't see Soh's face at the end again? Is the Yoshiro in NG+ different? I was thinking her face doesn't look like our Yoshiro...

Round...

This face is totally different to me.

Standard cute round face but it's NG+

Her face is not entirely consistent...but it might just be graphics. Her face looks rounder at times. Maybe her veil is throwing me off? It's probably just graphics and has nothing to do with anything...I don't know. I just want to present all my observations. 

This story is deeply touching, about a previous sacrificed Maiden wanting to break a cycle and not see another girl go through what she did. Love is a great motivator. But so is hate. Cursed Soh wants to end the cycle, too. By the destruction of everything, including Yoshiro. Which path to take?

Wanting to end cycles and the destruction or creation that can come from these feelings depending on how we decide to end cycles. Soh created something new...a new Goddess.

In my previous post where I mentioned the 'Soh as the previous Maiden' theory, I wrote:

Soh as the previous Maiden now guiding this generation's Maiden. She can only guide her to the same fate...until she puts an end to it. 'We must be better and do better'.

But it all starts with the desire for change which can come from something as simple as being very fond of someone dear to us.

But it's up to you to decide what the theme is. Art tells different stories to everyone.

Rapid Summary: Neither have a name. Yoshiro is the current Maiden but it's the name of every current Maiden. Soh is the previous Maiden. This is why their faces are different. Every Maiden becomes a Messenger to the next Maiden. We fight Cursed Soh who resembles Yoshiro because Soh was once a Maiden. The defilement that morphed into Cursed Soh was Soh's Spirit Stone body but is the result of countless girls being sacrificed this way. (possibly 7 girls actually) It's accumulated defilement across many generations.

Soh guides Yoshiro because she's walked the same path. This is why she never seems surprised.
Soh's possible fondness for Yoshiro brought out Cursed Soh who wants to end the cycle and decides to obliterate everything, including Yoshiro. Soh defeats her, purifies Yoshiro and ascends to being a Goddess. The Goddess has Soh's face and Yoshiro's accessories because Soh was a Maiden.

Extra extra!!

Getting the Cursed Soh skin is gonna be sooo satisfying. Redeeming my girl is gonna feel great. She just wanted to save Yoshiro and end the cycle. Even if it meant killing everything and letting defilement run over...

Well, if the Maiden is dead, she can't suffer. If everyone is dead, there's no more cycle of sin. That makes sense to me!

I might post a Part 2 once I hit NG+8. I'm learning a lot through multiple playthroughs.