There has not been, to my knowledge, that exact set of events you describe, but you cannot deny the weirdest things have happened over the years that would be unbelievable before it is proven.
The gal in Florida, next door neighbor, kept the gal alive in his sister’s house, then buried her alive.
The guy in Idaho that wiped out a family for the two kids.
We could go on and on.
The Ramsay case will go down in history as one of the great unsolved cases, with people on both sides; neither any more or less absurd in defending their side of the argument. Unless someone confesses that matches the DNA found on her.
No, I cannot deny it. I have never said that it's impossible for a total stranger to have killed Jon Benet, only that I am skeptical. As I posted earlier, being skeptical does not mean one's mind is closed to a possibility, only that one requires proof before accepting that possibility as fact.