If LE had evidence other than lawn guy's word that Terri wanted him to kill Kaine, I believe she'd already be in custody. A lot of LE's moves thus far seem designed to put the squeeze on Terri, but so far have yielded nothing. If they had evidence and were still trying to put pressure on Terri to divulge Kyron's whereabouts, arresting her and letting her sweat it until a hearing seems as good a way as any. Similarly, if LE had evidence Terri was directly involved in Kyron's disappearance and murder, she would've been arrested by now, and IMHO, they'd be cutting a deal with her to locate his body. The continued assertion by LE and Kyron's bio parents that he's alive tells me LE has no evidence a murder took place, and yet I think it's improbable he's still alive.
I think there may be an arrest this week, but at this point, it's more likely lawn guy will be arrested before Terri, IMHO. I honestly believe LE has very little in the way of hard evidence against Terri for any of this -- the murder-for-hire or Kyron's disappearance.