I believe KH and TH absolutely believe she is guilty. But I think KH and DY believe that because the police told them that THEY think she is guilty.
I have a lot of cops on my family. One a sheriff, patrol officers, detectives, and even MP. I can say without a doubt that they are good people, but they are just people. All police are just people. People are fallible. Policemen can "get a feeling" someone is guilty, they can be prideful and ignore anything that goes against what they think happened, they can be jerks, they can be wrong, and they can get tunnel vision. Even the most talented of LE officers can be susceptible to getting it wrong. Thats why there has to be proof beyond a reasonable doubt to convict someone of a crime for which they are accused.
As websleuthers we know the basics of an investigation. We know that when harm befalls someone the first persons to be looked at are those closest to the victim (barring any obvious evidence that pointed directly to a perp). IMO, because they are the most likely to do someone harm. Statistics and all... But to move on to the next layer of possible suspects cops tend to want to verify the family's whereabouts, their aliby, etc... What I think has happened here is that they can't get past TH because 1) There is no definative proof of where she was immediately following Kyron's last known sighting (which could happen to any of us), and 2) because cops have concentrated on her for so long they have developed tunnel vision. Add to that an alleged MFH plot and that tunnel vision has morphed into a perceived truth. However, I don't think there is any evidence to support their suspicions, just as there is no evidence to clear TH at this point. They are stuck.
If you take into consideration that most of us respect the authority of our law enforcement officers, the fact that one of the victims in this case is married to a detective, and that a precious child is missing you might see what I see. Most of us don't have the first clue about how to handle situations like this (in fact we must involve LE because they are the authority when a crime happens), so we look toward LE to be our guide. We don't imagine that they could be wrong. We don't question them. We hang on their every word because its the only hope there is of getting the missing child back. So that makes people vulnerable to be used by cops with tunnel vision to put pressure on their suspect. If the cops are right they are geniuses, if they are wrong they're just doing their job and TH was just collateral damage.