It's been a while since I've visited this case, and I could've sworn Kurt Sova had a much longer thread on this site—and that I even commented on it in the past—but I guess it's been deleted?
I don't know if I can add any new speculation at this point, but here are my thoughts:
I think the simplest and likeliest explanation is that Kurt Sova fell into an
alcoholic coma after drinking Everclear at the party in the duplex. Since he was still breathing, the residents assumed he had just passed out so they let him sleep it off in the basement in the cot.
Maybe they tried to care for him over the weekend, but when Monday came, the residents had to go back to work so they couldn't check on him regularly and he died. I don't think he choked on his own vomit because there was no evidence of this at autopsy, so I think he died from brain damage, organ failure, respiratory depression, or any of the other consequences associated with
alcoholic intoxication.
I think this explains the residents' ever-evolving story. It changed so many times as the situation deteriorated in the basement. The residents first said Kurt was never there because they didn't want to get in trouble for hosting underage drinking. They figured Kurt would wake up soon and they could just swear him to secrecy.
When Kurt wasn't waking up, they
had to admit he was at the party because by this point people were starting to say they saw him there. However, they tried to deflect attention away from the duplex by saying Kurt got sick and just walked off.
Kurt probably died Monday night into Tuesday morning. When the residents realized this (on Tuesday evening?), they were forced to come up with
something in case the police searched the house and found evidence of him. So they called very early on Wednesday morning and basically said, "hey, we
think someone's been sleeping in the basement" (an oddly specific detail). "Maybe it's Kurt; come get him!"
The original plan, made in desperation, could've been to allow Kurt's own family to "discover" him in the basement, with the residents pretending not to know how he got there. After all, they didn't
directly kill him so what could they blamed for? However, the holes in this story soon became apparent. If they suspected an intruder was sleeping in their house, why not call the police first instead of the family of some random missing boy you claimed was never there? So they decided to dispose of him quickly before Kurt's family could come.
I don't think there's any connection with Eugene Kvet.
That UM segment always stayed with me.