I agree that it seems most likely that it’s Louise’s room, though I’m still not completely convinced. If someone
really wanted to find out, I guess they could go back and check the colour of the bedsheets when Lena spies on Louise and Jeremy, plus check the windows to see if they match with the ones in Lena's and Stan's rooms and the living room. But that's more work than I'm gonna put in, at least…
When the original sketch was published, I think I commented that the doll looked creepy, but that it would probably look less scary after colouring. That was definitely wrong, and now there’s also two of them. Louise’s voodoo doll of Ivy, which she stabs with needles every evening? And one extra doll just in case she gets any more enemies?
A Lena, Louise and Jeremy threesome would be surprising at this point, in my view. Lena’s sex scenes with Louise only happens if she and Jeremy have broken up, and Lena’s Jeremy scene at Ivy’s apartment only happens if the two have not broken up – so there’s no way she can be involved with both of them up until now. I think this makes most sense if it's something Jeremy suggests, and Louise accepts because of her naive desire to satisfy him, and her (also naive) worshiping of Lena.
A Lena and Jeremy scene without Louise seems unlikely at that location – there’s no obvious reason that would happen in Louise’s room, even if she’s not present. Except maybe if Louise leaves the two of them alone in the apartment, like when they first met, and Lena decides to spy on Jeremy again…
The last potential scenario would be Louise and Ian, but I doubt we'll see that now. I think it’s a likely surprise scenario at some point, though – if Lena and Jeremy have sex and Louise finds out, I think it fits well with her over-the-top reactions if she seeks out Ian to have sex with him as revenge.
EDIT: Looking at the sketch sneak peek again, I do notice that there's only one notable change between them (in addition to the second doll): The additional bookshelve above the bed. That would mean that Eva probably has given a feedback along the lines of "It looks great, just add more books". That points towards Holly, even though the posters and other things suggest otherwise.
