I've been thinking about this a lot.
On September 10, regardless of anything else, they knew something had gone very wrong. They knew on the 10th because NPPD came to the door looking for Gabby and only found her van. We can argue and debate about what happened between the 1st and the 9th forever, but on the 10th Chris and Roberta knew something bad had happened in Wyoming. Brian didn't leave until the 13th. They had three full days to save his life by handing him over to the FBI, FL law allows them to have physically/forcibly handed him over. Because, yes, I think they knew he was suicidal or they wouldn't have been "looking" for him so soon after he went "hiking".
Maybe if they had hired a Florida attorney, Brian would still be alive to answer for his crimes. Folks, it's important to hire an attorney in the state where you reside and an attorney in the state where you committed the crime. I mean, the best choice would be to not cross state lines on a crime spree, but here we are.