Couple of thoughts
1. I understand why K would call J multiple times prior to passing out - drunk dialing, etc. However, I don’t know why M would as well, unless there was a reason why J would specifically not pick up for K at that time.
2. If the killer had been in the home previously as a friendly, he had likely been on the Wi-Fi and would likely auto connect when in the home. That data should be on their router and easy to obtain by LE.
3. IMO, the most rational explanation for the 911 call is that someone (potentially J’s friend) walks into the house and sees the bodies, heads downstairs and sees the surviving roommates. Asks the first one he sees to hand him their phone and tells them to get out of the house and then calls 911. That would put the other roommates out of the house at the time of the call, explain why one of their phones was used, and still make that caller seem suspicious enough for the police to investigate him.