Bestseller I don’t think she said to her friend she had killed Noah, it was very carefully worded “my baby is dead and it’s my fault”. He friend probably interpreted that as “someone took him and he couldn’t be found so he must be dead and it’s my fault for leaving the car”.
I remember Joanna had a black eye when arrested so I think her and Alistair had a fight, she killed him, and the truth about Noah has never actually come out. The public wouldn’t be calling for Justice for Noah if they knew that she had been complicit in his death being covered up.
I think they fought because Jo found out Al had overdosed him, and had kept her drugged and sedated so he could manipulate her and twist the story his way.
She “woke up” because it clicked in her that Alistair was a lying, manipulative, devious and controlling bastard, helped by Alex talking to her. And she was no longer sleep deprived from the baby, and had stopped taking the tablets Al was feeding her.
Alastair’s true side came out in his interaction with Alex, you got to see just how aggressive and vicious he could be.