Well let's say hypothetically they did find evidence, cadaver dogs picked up a scent in the house and in the trunk of the fathers car. They then confront the father with the evidence. They lie to him and tell him the wife told them everything. It leads to his confession. Even though they lied about the wife they are still ok because they have actual conclusive evidence that Joe died in the house. That would be totally appropriate.
Now alternatively, they just don't feel right about these people from the beginning. He's weird, shes a schizophrenic, he's not acting like he's worried enough, he's not trying to find his son etc etc... so they set out to arrange an interview. They lie to him and tell him they have evidence Joe was killed in the house and put in the trunk of the car. If he didn't do it then the mother must have. Who killed him? Father confesses to possibly protect the mother. Then they go to the mother, tell her he has confessed. They tell her the same lie about the evidence. She proceeds to try and figure out what must of happened and they start leading her to fill in the blanks. She's thinking he couldn't have but if he was killed in the house and it wasn't her and it wasn't him it must be the Grandparents?!? The police confront the grandparents but are ruled out only because they can prove they were out of state. This is coercion without evidence. That's not ok. If they do find evidence after the fact they risk it being thrown out because they broke the rules. If they don't find evidence at all and no body then the case is entirely circumstantial and I don't see it ending in a conviction at all.
Why do we still not have a body? I'm under the impression that during the initial confession the father did tell them where his body was but they did not find Joe where he led them to look. Now he nor she is cooperating. What logical reason would they have not to take them to Joe's body?
When I said that LE might have sensed that something was 'off' I didn't mean they found JD weird and his wife a schizophrenic. Someone who's schizophrenic and on meds should be fine.
What I meant was things like comparing the 911 tape to those made by people who were innocent vs those who were found guilty. Comparing their words and behavior to those known behaviors of innocent and guilty. Considering how they acted when confronted with the information that someone had driven down that road that night and saw something.
There is actually another piece of evidence that they may have been confronted with. It's been in the media that the grandfather says KD was telling people that "it was [the grandparents] fault"...what if she's said other things to people and those people went to LE with her words? Not everything has come out in the media, some things will be held back lest they be prejudicial in a trial, but sometimes people on here put up a link to a news story on FB and say "look at the comments by XYZ" and we're not allowed to discuss the content of those comments on the open forum, but those of us who saw them can't unsee them. So all I can say is that the grandfather's comment does suggest that KD does sometimes talk to people and say things and LE are going to want to know why she's saying those things to people at such a time. What if that leads to a confession?
We haven't heard in the media that the siblings of Joe have said anything, but that's not something that's likely to be reported in media, so they might have heard something that night that would give LE cause for concern.
The media says that JD confessed and said roughly where he drove with little Joe's body, but not a specific place. If only one of them was on that drive, maybe the other genuinely can't help with finding the body. I don't know the reasons why someone hid a body wouldn't tell as much as they could at this point. That little boy and his family deserve to at least have a proper funeral and burial.
LE, I believe, will wait for a search warrant before searching the home, but they are permitted to observe and see if maybe one particular room smells strongly of bleach or things like that, and the parents could be confronted with that.
There are definitely things that could lead to suspicion of the parents that are far more legitimate methods than the conjecture you have made.
In the video of JD and KD walking together outside the courthouse/jail, I thought he seemed to look over at her with a bit of caring in him as if he might have been asking her how she was? I can't really imagine him confessing on her behalf, but I can't rule it out.
Another thing I noticed was that JD said he was the one who got up to get the kids ready for school and 'noticed' little Joe was missing? Was that usual for them, or if it was just for that one day that he was getting the kids up, or if it was an outright lie to keep KD out of the narrative? Whatever the answer is could give insight into the dynamics of that household.