I would say with the phone evidence, the crime scene evidence, plus the security footage and the trash evidence that they have already put together almost enough for a case. However why arrest him for murder when they have time to try and find the body or what's left ? So no need to arrest him yet. Maybe MT has an alibi for the time frame of the murder but as an accomplice not so much in the cover up. She's facing serious jail time. Sounds like she already confessed (there was a leak she did) but Mattis doesn't know it for sure and the DA told her to keep quiet. If that's the case Mattis is screwed and he can't help her, therefore his position of strength is nullified. She's not going to jail if she can help it, she's not THAT in love imo... jmo. Patience is needed, they are building a case daily. Wouldn't be surprised if in the end they find more evidence and he claims self defense, she attacked him and says he panicked knowing it looked bad...
Confession -
Connecticut man's socialite girlfriend 'makes a confession' to police about his estranged wife | Daily Mail Online