The 'goodnight Snapchat' may well, from anything we've officially heard, have simply been a mid-evening Snapchat, opened at 10:00 pm by her boyfriend, who promptly fell asleep after a hard day's work.
I'm a trace skeptical of "late in the evening." Too many contradictions around it. Right now, here's something that I'd believe:
Dropped off at bf's home at 5:30.
Took care of dogs, and settled down to do homework, taking occasional social media breaks.
On one of those Snapchatted to boyfriend.
On another, texted to mom, checked that she bought ingredients for Friday after-work dessert. Was casually invited over to dinner, and equally casually accepted.
Finished homework, or got it under control. Put dogs in basement. Set out on a run that would take her over to her mother's place in time for a late dinner.
Vanished.