It still boggles my mind that the wife saw that the security system had been breached and didn't do at the very least do a perfunctory search. Mrs.Lone and I do that every night we go to bed just out of habit from when our two were babies and just don't feel comfortable if we don't at least peek in at them.
It is a shame.
You and your wife's habit is what normal parents do. These guys are not like most people I think. From watching them on the TV, I never thought that they killed the girl or had anyone do it, though. I haven't followed this case real closely, so I may be missing relevant info, I don't know. . .
I tend to agree with you that they probably didn't have anything directly to do with her killing, but they "seem" to know more about it than they are letting on. A couple posters here have seen some interviews with the parents and thought something just wasn't right.
Purely speculative of course, but being that they were apparently into some odd sex stuff I wonder if they made some deals while under the influence that may have led to this.