Yes, I generally don’t condone violence but if Warren had confronted Danny and punched him out, I couldn’t have blamed him.
One small thing from the book that goes against the idea that Warren knew or believed that Diane had a drinking problem: When Jackie first tells Warren that Diane had slurred speech and sounded drunk, Warren’s response was, “Impossible.” Obviously this is just my interpretation, but if he thought or knew she drank a lot sometimes, it seems as if his response would be something different, such as “Oh, shit,” or “I’m going to kill her,” or something that indicated anger instead of disbelief at the idea of Diane driving drunk. So I am really wondering if Diane was just a very occasional and low key drinker rather than secret heavy drinker, and that’s part of why the entire family was so shocked at the toxicology report. That doesn’t negate her decision to drink a ton of vodka and smoke weed, and drive, though, but it makes me skeptical that she was simply accustomed to driving while lit and misjudged the quantities she was consuming. Someone upthread suggested she went to the convenience store in search of Tylenol specifically because of the likelihood the combination of vodka and Tylenol would kill her, and I think that’s a good idea. I think she wanted to die and was past caring if she took other people with her.
Jackie mentioned in her book that during the roughest times between her and Warren after the accident, she didn’t feel she could leave him because he had been so devastated when his mother left. Maybe Diane also had such an aversion to divorce and breaking up a family that she decided death was better than being put in the same position her father had been, deserted by a spouse having an affair. And the father (the elder Mr. Hance) came across as not especially supportive or able to empathize after the accident, and I wondered if he was also that distant with Diane when the mother left and he became the sole custodial parent to Diane.
ETA: Regarding the impression that Jackie wasn’t especially close to Diane, from Jackie’s comments about her friends, it seemed like most of her close female friends were SAHMs like herself. So I suspect that Jackie and Diane weren’t as close in large part because Diane was the primary breadwinner in her own family. Between her job, taking care of her kids, and taking care of her man child husband, Diane probably had minimal time to socialize with other adults. So I don’t really see the closeness or lack thereof between Jackie and Diane as indicative of anything other than very different schedules. Which is not to say they would have been BFFs if Diane had also been a SAHM, but just that even if so inclined, Diane didn’t have the same flexibility in scheduling that Jackie and her other friends did, to get together for coffee or something during weekdays. Jackie did say at one point that she had loved Diane like a sister, but I would think any socialization would be on weekends. From what she said, it seemed like a normal relationship with a SIL, except it was very apparent that Jackie didn’t care for Danny. My impression was that nobody in Diane’s family liked Danny or had any use for him. OTOH, in almost any family, there’s going to be a token spouse or other in-law that everybody else can barely tolerate.