Is she usually so unreasonable? Doesn't she understand this kind of crap makes it look like you're hosting your own shower?
What's her reasoning on the return address thing? Can you use your mother's address or another friend's address instead?
If none of the hostesses are going to step up and and take responsibility for the RSVPs like they should, I agree that they should just use a generic email address with no one's name listed and you can check it if you have time.
Sorry you're dealing with this. If they don't want do deal with RSVPs, maybe they shouldn't have them at all. They can just get food for the total amount of people invited and deal with the leftovers?