I get where you're coming from. I think he knew he'd beaten Arthur, and only they knew about the isolation. I think he thought ET just went along with it. I do believe she pretended to be an innocent bystander. Ultimately though, it was her house, and as a mother herself she could have voiced her opinion strongly. I think she worked hard to separate Arthur and TH. And, life with Arthur wasn't part of her long term plan.
But, I don't believe he knew about the salt. And I think once he got a solicitor, and was made aware of the salt that's why he pleaded NG. Plus, he maybe wanted to protect her because she was pregnant.
I don't think any of us will ever understand how 2 adults could treat a defenceless little boy like this. My only hope is they get lengthy sentences.