I don't think a PI is a ridiculous idea, I just don't see how they can afford a decent one, if they are not working and the money coming in is for their survival, for one thing. They don't have access to the reward money for other uses, as far as I know. PI's won't work on a contingency basis, they want money up front.
I do think they should have made use of group such as Klaas Kids way back at the start, as soon as LE stopped their searches.
If LE has not been able to figure out who did this from her cell records, and whatever else they are keeping under their vests, I am not sure a PI could either. But of course, it would not hurt to have one , if there was a way to pay for it.