Not trying to be Freudian here but I think the whole issue goes to ego...Daniel needs to prove that the accident was not Diane's fault because otherwise it may indicate many negative things about him. Is he responsible? Probably not...but...there are a lot of what ifs...Maybe Dan drank a lot more than Diane so, to him, she is not a drinker, he is. So, maybe they both were drinking and smoking but him being the bigger drinker, he really doesn't see anything out of the ordinary with the amount she consumes (which he probably would have no idea of anyway)...maybe he drove home half bagged and really would have no idea of how she "seemed"...but isn't it so that you can be held legally responsible for someone leaving your home* intoxicated and then getting into an accident? I believe it is...so maybe in retrospect he is not so sure how she seemed and worries that he could somehow be held accountable.
Also, he doesn't want to seem to be what he seems to be...a husband who maybe really didn't know much about what was going on in his wife's life. Who didn't really know the pressure she was under and his part it in...Again, is he responsible, probably not, but could he have prevented it, not necessarily but possibly through different choices that day, but in the years, months, weeks, days, leading up to it. And then, there's that Thursday night thing...if they are fighting about whatever this was I bet it's not the first time. So, who knows how she's reacted before? (Daniel)...Maybe she's not an everyday drinker but at some points in time goes on a binge in reaction to something. He would know that and maybe now question why he didn't think out the morning and travel arrangements more carefully...Guilt is a huge piece in the puzzle of denial...Is Daniel responsible? Maybe not legally but ethically quite possibly...I was married to an alcoholic who also like to smoke pot...I would not let him drive my child anywhere after imbibing in either of those substances. So, maybe he feels he should have known better. As another poster said, maybe they were up all night and she was still under the influence...maybe he was, too.
I feel his defense of her is not really about her. He is defending his own ego...
ETA: I know they weren't home, but the legal implication could be similar...