I still don't think Misty was the tipster. Her own lawyer said that Misty's info caused someone else to talk (or something like that). I think bringing her out to the river was purely for investigative purposes. Take her out there and see what kind of emotions come out. But, who knows, maybe she is the tipster and she has grown a heart.
It may be that LE has been able to glean enough info from all those possibly involved to be able to go, convincingly enough, back to those they suspect may be involved and have convinced "someone" that the beans have been spilled....gig is up? I could see a female of MC's age finally cracking.....thinking someone else "talked". I do think there's more than one person involved with this though.