One of the main "feature" of eZp is that you can customize it the way you want to have it. It's not easy to do that in every cases, but it's possible.
You could then setup a simplified interface for those volunteers, with very limited functionnalities, but enough to do what they have to do.
If you want to stick to the default interface, you'll (IMHO) not really meet your first constraint: "can it do it in a way for users who expect the PC to opeate as easily as a toaster?", as the default interface is way too much complicated for a lambda user...
Finally, the superadmin could access this default interface to manage the entire site. But there are a few questions which you'll have to think about, like "do I want a validation system before publishing", for exemple.