yes, almost definately there would be of some kind, unless it was deliberately washed off to avoid detection. At a minimum, they would know whether or not they were strangled by a ligature or someone's hands. To my knowledge, there has never been an announcement they were testing dna found at the scene relative to the two "known" homicides. That in itself would concern me as in if they were so positive Lindsey and the baby were killed by the couple why wouldn't they disclose that?
My personal opinion is that they were strangled by a ligature of some kind and the autopsy supports that and the "instrument" is missing from the apartment. If that is the case, the theory of "it's an outside job" stands.
I respect your opinion that you feel an inside job is impossible, however, it is my experience in "crime land" anything is possible. I also know that in the past in an effort to not "tip off" suspects, sometimes LE gives either misleading or incorrect information that they are able to say later was not on purpose to manipulate progress in an investigation. This is not a criticism, they absolutely need to sometimes.