You will all be glad to know the areas you're referring to are areas I had already made clearer because my SO was complaining, so you're looking at the easy version.
So in regards to your questions:
Where to find the latex?
Where is the attic?
Where is the Shaman?

Where to find the latex?
Spoiler
There's an altar devoted to Steve's MDMA trip. It can be accessed from the upper floor of the blue house (It's white and ornamental, with carved rabbits. It's not a puzzle: just the second door on the left.
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Where is the attic?
Spoiler
The attic is part of Steve's Semi Intellectual Sentiments (that would just be the blue house), and it can be accessed from the bedroom (the room with the bed, broken television and the Duchamp painting on the wall.). There is a puzzle to it, but it's not a hard one, and you don't need anything outside the room. The attic is made of wood, and quite a lot players already found it, so you should be able to locate it. The attic has the weapons room, btw.
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Where is the Shaman?
Spoiler
The Shaman is hiding in Steve's subconscious, which is not a "physical" plane, and cannot be accessed through a door. In order to see the Shaman, you'll need Syph's help, a way to shed your body and a backup. It's not an easy puzzle, but it's the one that concludes the game, and several people already solved it.
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