Actually, I'd say you just described a lot of incompetence - if they allowed the DNA evidence to become contaminated, or misinterpreted evidence, or miscommunication. Competent cops don't allow those things to happen, IMO.
I think it would be helpful to walk through a scenario involving faked or planted evidence. This is the way my mind works, I can't just say "anything's possible!" without thinking it though. Can we explore the logistics of the hypothetical staged death scene? Because I believe that, when we think it through, it's a very implausible theory, at least based on how I would imagine a staged death scene.
The problem with a theory that involves the staging of evidence to make it only *look* like the Ls and NO are dead, as I see it, is that we would have to assume the evidence staged would be blood. AL and KL withdraw and store their own blood over many weeks or months, until they have a sufficient volume that, when spilled at the crime scene, would cause LE to conclude they had died. I'm sure we've all seen a movie or tv show where someone faked their own death in this way.
Problem: Why would they allow NO to spend the night, on the very night they plan to disappear/stage their murder? How did LE come to the conclusion the NO was dead? Did LE say "well we have lots of blood from 2 out of 3 of them. That's enough for us to conclude NO is dead too"? Did the grandparents wound their grandson and cause him to bleed so profusely that LE would also conclude that he was dead? And then kidnapped their grandson? It doesn't ring true.
Is there another way to stage a death besides through a vast volume of blood evidence?
And then, how did DG come into the equation? What are the chances that he, a relative who also had business dealings with AL, was seen in their neighborhood on the very night they staged their disappearance? It's too coincidental, IMO.
Do you have a plausible staging scenario? Anyone?