IMHO this is definitely plausible, and probably more plausible than some of the other theories out there. I've always thought something like this was a possibility from the beginning of this case, but then I watch a lot of crime drama TV shows/movies - so that may be affecting my view here.
The fact that the perp. came when everyone was asleep, committed the crimes, and left the house without alerting the 2 survivors (and, no - I don't think they had anything to do with this) seems more like a professional, methodical hit - than that it was done by a crazed/spurned lunatic/stalker who couldn't contain their intense rage.
That being said, if it was a hit I somehow don't see all 4 victims being targets. Maybe 1 (or 2) were targets, but the non-targets were killed because they were either in the way and/or because the killer didn't want witnesses?!
If this was the case, maybe the reason the two survivors on the bottom floor were spared was because the perp. didn't have any reason to kill them - given that they asleep; weren't alerted to the crimes; and didn't see the perp. But, it's also possible the perp. didn't know they were in the house.
However, this is obviously sheer speculation & I could be very wrong.