SBM
This is the biggest problem I have with this case. If TS was so bothered by Kortne's stomping (or whatever was going on upstairs) that he actually called the police, why did he not answer the door when they arrived? And is it just a big coincidence that Kortne also did not answer her door either?
The neighbour certainly wouldn't have answered his door if he had just attacked Kortne, especially if she were seriously injured inside his apartment. And he'd feel safe knowing that even if LE went knocking on HER door, she wouldn't be able to answer.
I suppose it's possible that Kortne had left her apartment and gone somewhere else by this time, but then why wouldn't her neighbour have answered his door when LE came knocking? It seems like too big a coincidence that Kortne seemed to have left by the time LE arrived (because I doubt she'd be asleep after getting so worked up) and the neighbour didn't hear them at his door.
I'm leaning more towards the neighour being involved than I am Kortne leaving the apartment of her own accord and meeting trouble later that morning; however, I believe that these are the two most likely scenarios with the limited information we have.