So prosecuters thought that the jury would believe Karla hade some kind partial amnesia due to PTSD

that only seemed to occur when it came to details that made her look guilty, like bringing home a victim and drugging her on her own, without Paul.

But the two people who kidnapped and raped and abused the girls (shown on video) also being the ones who killed them was too much of a leap?

It seems to me that Karla's partial amnesia story was a lot harder to believe. She had a strong motive to leave those things out and remembered most of the details. However, it's highly likely that the kidnapper, torturer and rapist also killed them, even without physical evidence.