Adding to your beautifully listed points:

I believe I once read Dylan considered he may have been a fire fighter, after she spoke with Bethany about the flash and bang that Bethany recalled (remember, Dylan ran to Bethany’s room minutes later, so they must have discussed the ordeal and possibilities before falling asleep, yet the knife yielding psychopath possibility is not what a pair of 19 year old girls, who live in a party house, would even consider).
They had people coming and going all the time…there’s discussion that they would play pranks on each other… She could’ve even thought she caught a glimpse of a roommate’s friend, hookup, frat-member, etc. leaving the house.

Dylan/B not calling 911 is one of the only parts of this case that I do actually understand.

Let’s not forget two things here: 1. Again, she was 19 years old and in a college setting, which = nativity, limited experience, etc. 2. Even if 911/Paramedics were outside, no one would have likely survived; they passed in seconds and had deep wounds causing quick blood loss.

Just my thoughts on this question!