Mary is in The Dark Place
Going off that, Mary is the only remnant of Barbara Jagger's soul that still exists. The monsters in the house are vessels of the Dark Presence, designed to psychologically break her by feeding off her insecurities before wiping her out completely. The woman in white is Alan, who wrote himself in like Zane did but changed some details (such as the character's gender) so the Dark Presence wouldn't get suspicious, as he is supposed to be writing for the Presence, not against it.Mary is struggling with depression and drug addiction
The entities that torment her in the house every night are manifestations of her depression and "The Garden" she seeks refuge in represents the drug she's been self-medicating with, namely heroin. Mary may very well have taken a dangerously high dose of heroin and is in danger of dying unless someone saves her.Mary is a transwoman
If the Mirror Monster is anything to go by, then Mary has strong body image issues. Furthermore, the Mirror Monster is called "Beauty" by fans, she could represent how she wants to be perceived, especially since in an early episode Mary rips the head off a photo of herself, saying she doesn't look good in the photograph. This could be reinforced by the fact that the Mirror Monster wears a dress and laughs at Mary for trying out a new dress in an episode. Furthermore, it's self-evident that the other monsters, the Veiled Woman especially, seem to represent Mary's anxiety, which could represent Mary's desire to be perceived as a woman, yet nervousness about being judged about who she is. In summary, the monsters could represent the dysphoria, anxiety, and depression many transgender people feel.The house is located in Silent Hill
K, the creator of the series, has cited Silent Hill as a creative influence. Given the fact the house and monsters reflect aspects Mary's psyche, like the locations and monsters in the video game series, I think it's reasonable to assume that the webseries takes place in the town of Silent Hill itself.The series is a protracted Mastigos Awakening.
We're secretly seeing a bit of Pandaemonium that Mary's been trapped in, with her doubts and insecurities being externalized as Goetic demons, with the Woman in White being a Supernal daemon who recognized the pain she was in and decided to help her help herself, with the reason Mary couldn't understand her being that she speaks in the High Speech. Notably, her taking control of the darkness, she's able to reconcile with her own regrets in the form of the Shadow Man, and banish the bit of her that's Ahriman; the Veiled Man.The series is connected to The Slender Man Mythos
The knocking on the door in the video "last 8 months" is revealed in Stan Frederick, a series connected to the Slender Man and other characters within its lore, to be partially caused by characters from one of the episodes. If this is true, then Mary might have literally been trapped inside the house by either a curse or possibly Slender Man himself or some other entity, and that the monsters plaguing were very real and tried to drive her insane or to suicide. If the Slender Man was involved, he could have been trying to turn Mary into a Proxy with Beauty/Mirror Monster being possibly what her Porxy form could have looked like.