Maybe they were angry when they found out that LE had been holding information back from them. And they were thinking about taking legal action against LE, or at least wanted to know their rights to be kept informed and wanted someone to advocate for them.
The parents seem to have completely accepted the fact that their daughter is no longer alive... They must have been told something pretty compelling to extinguish any last glimmer of hope. I have read about other cases where remains were found but were unable to be identified by DNA, eg if the body was burned and only bone fragments were recovered. Or maybe an informant told LE that Brittanee was held in a certain place and they found her blood there...
I can't imagine that someone's word would be enough to make LE and Brittanee's parents so certain that she is deceased... The informant or informants could be lying, after all. I think there must be something more. If not blood or remains, maybe CCTV footage... Or a photo or video of Brittanee's body.