Re the ferocity of attackI can't believe that if these 2 are guilty, they could have just thrown away their own phones as well as Brianna's and disposed of the knife and claimed they went off for a walk alone and came back and found her dead. They could have said the blood came from them touching her body. I think without the texts and knife, their involvement would be a lot more difficult to evidence (if guilty).
Also, the 28 (I think it was) stab wounds seems to have been brushed over. I mean this would have taken a huge amount of physical energy and god knows the psychology behind it. If these 2 are guilty and had planned to kill someone potentially for the first time, I would have expected one or two stabs and then them to run away before they were caught. I don't feel the ferocity of this attack has really been addressed. JMO
I guess that whoever did it, wanted to be 100% sure the victim was dead.
As I wrote earlier, the perp/s must have been in a frenzy

Such a state of mind gives extra physical strength.
JMO