If I may ...???
When it comes to YOUR emotional health, yes, you are overreacting imo. This is affecting you so negatively. To me, that fact means that you have taken this far too personally. If you can step back away and realize this really isn't about you...though you are involved... you will get through these next two weeks much easier. The reactions you are experiencing aren't worth your health.
(((hugs)))
While I am a "career" volunteer, and know that it takes a special "hand" to work with volunteers, it hardly excuses any bad behavior upon any volunteer's part. But with that said, it is part of understanding the mindset of the average volunteer. It [i]shouldn't[/it] have to be the way it is; ppl should treat each other with courtesy and appreciation no matter whether paid or not. I'm sorry you have come toe to toe with some poorer representations of the volunteer force.
In reading, I wasn't quite sure WHO is over these volunteers. If this is a joint effort, then the heads of each group needs to deal with the problem, not you. If there is a coordinator over the volunteers, then that is who should be advised of the problems at hand. If they happen to be the head of the volunteer force, then the relationship needs to be discussed and possibly reevaluated by the university? As I said, the care and feeding of a volunteer work force takes special understanding, and as you may have found out, patience.
I'm sorry that you have been treated badly. At this point why not jot down the tasks to be finished (if you haven't already

) and think about what alterations might occur and how far of an adjustment is possible. That way if they throw you any more curve balls, you'll be ready. Instead of having to cowtow to their misunderstandings, you will then be able to stay professional and also regain your patience and understanding.
Breathe. Nothing they do or say diminishes you in any way.