Sorry to hear about your horse. Isn't it strange what the mind does? I agree with all your points - and even if DM didn't know the food had been ordered, she would have been quite familiar with food orders. I agree she probably thought the guy was the delivery person, although they do not usually come in the house, she may have thought this night was an exception or that someone else in the house knew this person.
Our minds are not at their best in the middle of the night (usually, if we are ordinarily keeping a night sleeping schedule) and being awakened from deep sleep is a state where many people are not fully functional.
That's why this killer chose that hour of the night to strike. The witching hour. He was amped up (whether on his own brain chemicals or something else) and he knew the victims were highly vulnerable. I think it's possible he had more than one house as a target and was deciding which one was most attractive (e.g., most vulnerable and containing the right type of victim).
My big question now is why he decompensated/acted out to that degree on that particular night. Was it becoming more clear that being a grad student wasn't going to be a walk in the park? Was it the first time he'd received criticism from a professor? Was he so lonely and homesick (his first time away from his parents, that we know of) that he wasn't sleeping and was morose?
Your last paragraph raises a huge question.
Given his purported heroin addiction and the strong possibility that he's the author of the blog posts with his same date of birth, it seems likely that he struggled with his inner emotions and fought his personal demons for a long time.
And, his strong interest in criminology from a young age could possibly suggest him wanting to better understand his own frightening thoughts and urges.
IMHO, he has likely fought these urges for quite awhile. So, as 10ofRods asks, why he decompensated and acted out to that degree on this particular night?
Given that he was likely near that house for at least 10 times since August, he may have been having these horrifying thoughts about murdering one or some of these girls for almost 3 months but, if he had those thoughts, he didn't act on them until that particular night.
If this had been a long-term plan, surely he would not have used his own car or had his cell phone turned on during all of his purported stalking of one of the girls or several of the girls at this house.
Or, did he on some subconscious level want to be caught so that he could be jailed and no longer have to fight his inner demons and urges to kill others.
Wondering what factors may have led to him committing these murders on this particular night . . .
And thinking that if we could better understand how he ended up committing these murders on that particular night, perhaps we could more effectively help people who have similar inner struggles and prevent more of these kinds of horrific tragedies.