why did Alistair suddenly fess up to Jo when he is usually a quick thinking liar? Surely what he did was terrible but it wasn’t a cold blooded murder so did he really deserve to die for it?
It wasn't a cold blooded murder. But it was a cold blooded, 'lie about his own responsibility for the death, purposefully accuse Jo (he could have left it vague), pursue custody for his daughter at Jo's expense, lie to her about the burial instead of face up to his guilt like she did every day' kind of thing.
Jo wanted to die after Noah died. Then she wanted to confess. And she wanted her baby to be buried somewhere beautiful. He denied her all of those things and lied to her in the process, over and over again. Not a hanging offence, no, but more than enough to push a grieving mother over the edge, right?
-Why does Jo find contentment/closure in the house where her baby is buried underneath? Wouldn’t it cause distress to realise that your baby was under a building site?
Again, she just wanted to be near him. She asked for that again and again. We saw that in all three following episodes. She didn't want to be exonerated - she didn't even want Alastair vilified in death. She just wanted to be close to her baby.