Since the numbers of posts of the theory thread don't work anymore because it was moved, i'll leave my theories here, which are just my way to see the whole game and all it elements. Maybe it's a summary of all the theories made before, but i've waited until i could see the ending of the game to make up mi mind about them.
Sabitsuki might have a disease called "Hermansky–Pudlak syndrome", just because she seems to present the same symptoms of it. (To me, she is not sick of Cancer). This illness can be detected because of the hemorragic events that may occur, and also, it is visible because of the albinism, which that people suffer from their birth.
Sabitsuki maybe found herself in the flow state (psycologicaly talking, i read that a long ago) looking for a better way to live with her sickness, and that's why she gets all these effects, which mostly alterate her body.
The Uterus area is, to me, the Sabitsuki's mother's uterus (the way she imagines it). Sabitsuki was born with that disease, and The three headed "monster" looks like corrupted sperms (and freaking scary). Maybe Sabitsuki consider herself one of these defectuous sperms, because she was born with that disease.
Since i saw the original art by lol, i might presume that Smile and Sister are related to her, those are her brother and sister. Maybe they were a family of three children. (See the Three head monster, which can be related to three sperms) Sister is afraid of her Sabistuki because of her disease. Smile maybe tried to comfort Sabi for some time, but when they went to school, he left her because he considered their friends much more important. The one who made her "smile", laughted at her. After that, Sabitsuki moved alone, and lived like that for some time, totally isolated.
The kaibutsus don't have the same disease as Sabitsuki, they are just like other albinistic people. Sabi can't be their friend because they make clear that she's very different from them because of her disease, i think that's the reason why they leave her isolated, and also, laugh at her.
Oreko is a friend of Sabitsuki (maybe the only one), but she's afraid of Sabi's disease too (that's maybe the reason why she doesn't reveal her face). Although, Oreko looks for a way to cure her friend, or maybe look for a way to make Sabitsuki's life a little better (machine effect is kinda related to the big machine where Oreko's usually working).
There are two hospital parts. The one related to Sabitsuki's past (the one who leads her to the uterus area) and the one related to Sabitski present. She was looking for a way to cure her disease, and she blames the doctor and nurse because they can't cure her or their negligence of letting die many patiens of their hospital (the bloody arrow that points the room when they are made me think about this). The flowerish part are other part that show sabitsku was lookin even in a kind of alternative therapy. (The corrupted kids with blood and flowers are related, someway).
The meidobot are maybe people who wanted to help sabitsuki too, trying to comfort her somehow (the bar ones) or maybe convincing her to get amputated her legs to keep living. (Tumbler doll effect)
I still have some minor theories about all the other areas and i sure want to make a doushinji of .Flow about them.
The three endings show the desitions of Sabitsuki:
In the first ending, you can still see Sabitsuki, if you don't start the Rust Chapter. She decides to let herself die with the disease, still with the hemorragy. (the blood stains near the black-haired girls)
But, if you decide to contunue to the Rust Chapter, there is a kind of "story" in it. After some time, Sabitsuki got tired of trusting all the useless "effects" and therapies that didn't cure her, and dropped all. Then, she became Rust, and alter-ego of Sabitsuki which leave herself to corrupt.
In the second ending, when the encounter with the meidobot happens and she carries the dead body of Sabitsuki occurs because she decides to accept the operation or surgery to cut her limbs and after that just die, abandoning all.
But, in the third ending, Sabitsuki and Rust have different endings. Rust dies, and is carried away by the Meidobot. And then, a limbless Sabitsuki keeps living, inside the flow, bringing back her memmories repetedly as if there is no-life after.