I really want to believe they had no clue until they started receiving phone calls, the fact that there were two reports in the NPPD records for 9/10 (albeit heavily redacted) makes me wonder if there wasn't some kind of disagreement on what to do moving forward.
However, there are just so many unanswered questions, that may always be unanswered, that do cast a little bit of doubt there. I have said before, if they truly had no clue prior to receiving the calls and their behaviors/actions/lack thereof up to this point are simply them following their lawyer's advice because that's all they could do, it does make me incredibly sad for them. Unless BL came right out and told them exactly what happened to GP, I can only imagine they've been holding onto the hope that something would come to light to clear their child.
Even with the doubts, though, I always go back to: If you honestly knew nothing, did nothing, and were completely innocent, why wouldn't you volunteer everything you knew? Would you not want to get your name off the potential list of suspects ASAP not only for yourself but so LE could focus on finding the actual person responsible?