As far as a Bella/Jill past homosexual relationship, I see no indication of that (beyond theories and wishful thinking). The "partially spoken" clues point more likely to Bella apparently expressing interest in Jill's past romantic interests. The line "I won't let that happen again" and Bella's several private thoughts seems more likely to be alluding to Jill's observation that Bella is attracted to MC which we note in the scene where MC is playing guitar for the two and Bella gives him that SUPER CREEPY look, that Jill notices and is bothered by, hinting that Bella has done this before. In context, their conversation later and Bella numerous thought scenes, that makes sense, and opens credence to many of the theories to something strange if not out and out scary with Bella and the "Door".
Lots the conversations and thoughts of Bella in particular make no sense if the "mystery element" is just the two being romantically involved, and...frankly...was already done with Josie/Maya, so I doubt DPC would plagiarize himself in a single story.
My unspoken but Strongly-Hope-Isn't-True-So-I-Won't-Mention-It (but the update is about to come out so I'll just say it) is...disturbing as it sounds. Based on Bella thoughts, Jill's cross eyed looks, MC's peaking under the door, Jill tempted to try the Door, and the disturbing words of the narrative/MC after kissing Bella in the library: Does Jill have an unspoken suspicion that Bella "did something" to someone Jill was attracted to? Is that suspicion what is the source of the tension between the two ladies? Did Bella just try to steal a love interest of Jill, or was it more nefarious? Did she drive someone away? Did she hurt someone? Is she holding someone behind the Door? Is it someone Jill cared about in the past? I HOPE I AM WRONG! But that's my (until now) unspoken theory. All the clues point to something creepy and scary. But I HOPE I'M WRONG. PLEASE MAKE ME WRONG, DPC!
Food for thought. All theories are just theories at this point, but we'll find out soon enough.....
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