I also used to signal right with my left hand up, but I switched to using my right hand about 5 years ago. Like NY Biker, I also am very unstable taking my right hand off of the bar. In the beginning of a ride, I can barely do it.
In the instance of the person passing on the left when we were leading the group, I think he or she was just pissed at seeing another group of cyclists. There were several special things going on around here that day, cycling, running, and one of the main streets was closed for electronic recycling day.
Yesterday, well, I think they are not used to seeing and dealing with cyclists in the way they are here. Not that it's an excuse, but I really think some people don't know we are supposed to act like cars or realize how fast we can go. In both cases I think DH should have called the police, especially yesterday, but he didn't want to spend the time.