I thought about this too and we may never know, but this is my pure speculation. I agree with 1-3. After that, I think X may have heard something upstairs but she wasn't sure what it was. We know both M and K had boyfriends. We know they were very social, they went out to bars that night, they were chatting at the food truck.
Maybe X heard something upstairs but first thought it could just be a friend or a late-night visitor for M or K. X heads back into her room and that's when she says "there's a man here" to E (or just outloud). BK coming down the stairs hears X moving around and walks down the hall to their bedroom where he encounters X - either in the door frame or in the room.
I don't think X/E was BK's target so it is quick and then he turns to leave.
The one thing that I am confused about is if DM heard crying from X's room as she saw BK leaving. As in, BK left while X was still crying. Makes your stomach turn thinking about how unnecessary and cruel it all is. Just pointless evil.