I guess when I look at these unsolved cases I do it in a different way. I try to narrow the list of possible suspects as much as possible. If something doesn't make sense, like a random perp getting into the house but putting the screen back on the window, then selecting the right room that the baby is in, then taking three cell phones for no good reason, and just happening to be doing this on the perfect night when the father is gone, I RULE IT OUT. When I rule out something I shouldn't, it doesn't sit right in my thoughts and I end up putting it back as a possibility. In this case, it feels completely correct to rule out anyone who didn't have intimate knowledge of that house, where Lisa was, and the circumstances surrounding JI's work.
That narrows the list down quite a bit, and thankfully so. You see, if a detective doesn't do that, if he is a fence sitter, the case never gets solved.
Granted, we don't get all the information or see all the evidence, so it is harder to make decisions and solve the case. But that is why I am on this website, to try and solve the cases I post on. And I feel very comfortable narrowing the list of suspects to a dozen, and I also feel very comfortable that LE has indeed narrowed it down to this couple, even though it is based on JI's words. MOO.