I think US must have known at least some of what was going on. I can't speak to experiences of growing up with a dad working as I never had one but certainly the father of my sons worked very long hours when they were young and didn't always see them in the evenings but he saw their 'debris' - clothes in the laundry bin, school books, stuf they'd left in the living room, he'd see them on their way to the loo or hear them/see them in passing in the morning....tbh if there'd been evidence of blood, bloody bandages, or they'd been limping or shaky or unwell he WOULD have seen. With young kids, especially active young kids, the evidence of their simply existing in a house is everywhere, not just by sight but by hearing - we could always hear them thundering on the landing and running up and down upstairs (why ARE boys so unbelievably heavy footed lol), talking etc.
For US to argue he knew nothing, for me is about as plausible as Rose West trying to argue that she had zero knowledge of Fred West killing and dismembering 12 women in their basement - it's like nah darling you will hear and smell something even if you are literally walking round with your eyes closed.
I do agree that BB was the ringleader and would not be surprised if she devised some of the more esoteric tortures like the radiator and the hood, out of jealousy and her own evil sadistic nature. As I said before I reckon it will come out that she consumed a lot of media about tortures used on Guantanamo detainees and other war crimes and tried those sick ideas out on Sara. But US is NOT an innocent either!
The account of Sara's dying moments is horrific. She died not in the arms of loving parents but the arms of their abusers. It's horrible, horrible, horrible, these people are LOWER than animals. Especialy BB. bleeping evil manipulative sadist.