Mr. Z didn't have to buy into her testimony, IMO. He was never going to vote DP, and seemingly wouldn't have even voted her guilty if he'd had other like-minded jurors to rally around his cause.
As for the other jurors. It was the absence of criticism of LaVa that was telling. Very few if any WS's could abide her. No final juror that I've heard on the record expressed any such like opinion of her.
I get what a good expert witness is supposed to do. Its possible, though, to be completely professional AND approachable to the jury.
DeMarte's job this time around is different and IMO even more important. She needs to convince the jury that although JA has BPD - in lay terms, something about that girl isn't quite right, it is equally true that BPD is not a mental illness, that it doesn't make one violent, that given the 1,000 mile distance more than BPD was involved in why she chose to murder Travis. In short, yes BPD, but no, not mitigating.
Assuming jurors don't buy into the DT's stealth argument that Travis deserved to die, JA's psychological problems or lack of are key to determining whether or not she deserves mercy. DeMarte needs to be really really good to squelch any notions of pity.