Please be advised that none of the following is intended to be insulting or accusational to the family, I am just offering alternate theories and ideas that no one has mentioned.
There are lots of details that really stick out in my mind, and I'd like to introduce them here and get your thoughts on them.
1. MI seems very, very immature for her age. She was still living at home, and the mother, thus far, hasn't mentioned anything about her having plans of leaving. Her texts and conversations with her parents show that she was really clinging to them, more than the usual 20 year old. Was she looking for something in them? The photos of her room show a scene more likely found in a 12 - 15 year old's room than a 20 year old college graduate's room. She was eager to finish school and go to law school, but she was only taking two ballet classes that semester. Something doesn't add up here. She wasn't moving forward into adulthood, she seemed stunted. Was it her who didn't want to grow up, or was it her parents who didn't want her to grow up?
2. The CO poisoning. The documents that I have read have said, "Suspected CO poisoning." I don't see anything, aside from the mother's words, that MI was actually diagnosed with CO poisoning during this time. Was this incident another example of what we may be seeing here -- a small thing snowballing, with MI getting a lot of attention from everyone around her? Everyone worrying about her?
My theory right now is that she had a lot more going on than is being shared with the public. Perhaps she didn't even tell her parents what was really going on, perhaps she didn't tell anyone.
I'm not saying this whole thing was intentionally fabricated. I do believe that she heard some tapping on her window, or thought she did, but then I think maybe everyone started to hear things and perceive things that weren't really happening, due to their heightened senses.
In a case like that, it would feel very, very real to them. And maybe some of it was. Once the word got out that there was a stalker, maybe some mischevious people thought they would "mess" with the family more.
As far as the photo of the stalker, it could have been anyone. If I saw a bunch of police officers around my neighbor's home in the middle of the night, I may go have a peek, too. Perhaps the person was just curious and wanted to looky-loo.
Nothing is conclusive in what she has presented to-date, except I do not think there is a human alive who could perpetuate something like this with zero solid evidence left. Also, the death part does not make any sense for a stalker. Had this been a stalker, it would have gone down much, much differently, as stalkers are motivated by very specific things.
This leaves me with two suppositions regarding the death -- either MI did intentionally OD, or perhaps only meant to "slightly" OD and be rescued, or someone inside the home did this to her. Whoever did it did not want this to be a painful death. Whoever did it, if it wasn't her, really loved her.
Again, I know this post may be unpopular here, but I don't think we can figure this out until we consider EVERY possibility, even the ones that seem unimaginable.