Question: Were Sheila, her kid, and Diesel
...somehow related as Mad Dog really struggles to tell Diesel about them?
Sorry to ask, I didn't read the book.
"If Mad Max Fury Road is an 8, then I'll use 8 for OK, 9 is better, 10 is best."
No, he struggled to tell about them, because Mad Dog actually loved Sheila, but killed her anyway. He was ashamed.
Then, he kept going on an on about it, and Diesel got annoyed. Just like Sheila took it too far with Mad Dog, now Mad Dog took it too far with Diesel. Diesel said "you don't know when to talk and you don't know when to shut up".
Diesel just wanted Mad Dog to shut up, but he didn't. That plus the stench of the dead bodies drove Diesel to kill Mad Dog, in a place where nobody would mind another dead body anyway.