I'm trying to imagine how Jason would react to all this if he were innocent. Keeping Cassidy isolated or protected makes more sense if he's innocent. I don't expect Jason to be searching for anyone that may have done this and it does seem sensible to let the police do their jobs. He is a computer geek and knows nothing about crime solving. His lawyer has obviously advised him to keep quiet, and he's listening.
There is no question that the police are trying to build a case against Jason so regardless of what he says, it's going to be interpretted through the lens of 'guilt'. Speaking with the police may give them some information, but more likely it will give them some reason to arrest him. False arrests and false convictions have occurred, so Jason needs to protect himself.
As for Cassidy visiting with Meredith alone, that shouldn't really be necessary. If the Fisher family wants to visit, they should be welcome to visit Cassidy in her new environment, read her a book or do a simple craft with her.
I have suspected Jason from the time that this hit the news, but I don't want to take everything he does and interpret it as another reason to confirm that he is guilty. I like to look at the facts and wonder what they could mean. I can understand that he and Cassidy would keep out of the media and the public eye, that Jason would let the police do their work and that he would be afraid of talking with the police. If he didn't commit the crime, then he probably believes that there is nothing he can contribute to solving it. If the police have questions, they could certainly pass them along to Jason's lawyer ... and maybe this has happened. Maybe Jason is answering questions that his lawyer approves without speaking with the police.