I haven’t encountered Box & whisker plots or histograms in my work, although I am familiar with them from coursework. I think that once you know what you are looking for, they are easy to understand.
1. The best graph would be a line graph with points.
2. Yes, I would use vertical bar graph to more easily compare subgroups.
3. If I was just showing overall Eastern and Northern membership trends, I would use the line graph.