I agree this is a valid interpretation of "if" ... "then" in terms of possible targets and the killer(s) movements.
There are so many different ways of interpreting the possible (1) order chronologically of the 4 murders, (2) route followed through the house by the killer(s), (3) the number of assailants, (4) reasons behind the order or route followed or number of assailants...
The unknowns are so many, and in the "heat of the moment" the "best laid plans" of a person(s) who is intent on killing can change rapidly, and they just can react to the immediate circumstances.
Another possibility is that X and/or E on the second floor were the target(s) of the killer(s) and while they were being stabbed, K and/or M heard or saw something from above on the third floor that alerted them to trouble downstairs, and that is maybe why they made multiple calls to K's ex boyfriend according to LE (that went unanswered) during the timeframe LE think the murders occurred. So the killer could have gone up to the third floor to kill K and/or M after X and/or E because of what they heard or saw.
LE has a wide variety of forensic evidence to help narrow down the possibilities, it's just a matter of time for them to process all the evidence and data to home in on the most likely scenario(s) and suspect(s).
I think we all can agree on one thing, and that is that LE identifies a suspect(s) and there is resolution and justice for the victims and their loved ones ASAIHP (as soon as is humanly possible).