I have to be honest, I have ridden this entire spring/summer without a single incident of a bad motorist. I ride about 150 miles a week. There were a couple that were questionable as they didn't give very much room when they passed, or the guy who followed me for a couple miles when I was doing around 35 on a downhill stretch, but I chalk those up to bad driving or curiosity, respectively.
I think that if you look for bad behavior from motorists, you will find it. I am not saying that there aren't jerks out there who are outright aggressive with cyclists, but I think much of it can be shrugged off.
As for the guy that buzzed OP's group and was yelling at them, I would have stopped and found out what his problem was. I would have also invited the man into his life. If someone really wants to get that offended by me being on the road, then they probably need a chat with the police. If his buzzing you was intentional, then that is reckless driving. When I was a patrolman earlier in my law enforcement career, I took people to jail for crap like that. Although traffic infractions (like speeding) cannot be enforced based on someone else's word alone, more serious traffic misdemeanors can be provided there are good witnesses (and perhaps statements from the driver) to establish probable cause. In the op's case, that would be the aggressive act of buzzing the group as witnessed by the group, followed up by the guy yelling at you. Stopping and finding out what the hell his problem was would probably seal the deal as he would be likely to make incriminating statements.
Rolling coal? It happens. The black smoke only lasts a few seconds and its over with. If it were a daily or frequent occurrence with the same driver, just get the plate and report it to your local LEA. Otherwise, shrug it off. The world is full of jerks who try to get a rise out of people. Best thing to do is to just keep pedaling like it doesn't even phase you. I may be stereotyping, but the people I usually see rolling coal are obese neckbeards driving a POS truck that they had chipped at the local diesel performance place. I can't help but think that their act of rolling coal on cyclists (and runners) is done out of disdain for someone who is exercising because of guilt they feel for indulging in their 10,000 calorie a day diet and inability to reclaim the athletic prowess they once (thought they) had on the football team when they peaked in high school.