Sorry to crash, but I love threads like this
One thing I would advise would be too keep this stress away from the bride. While the bride has a right to know that the MOH is being a Beeeotch with a capital B, she's got enough stress going on with planning the wedding.
The MOH sounds like a pushy control freak. I agree with whoever said above that she's nuts for wanting to buy all the paper stuff in Hawaii and bring it. And what happens if she says she wants to do all this stuff then bails and doesn't even come to the shower? Then you're left up the creek without a paddle.
I would probably go with cutting her out of the emails. This you may want to try to explain to the bride, "Look, Suzy Q is not being helpful and is making everything into a crazy stressful power battle. I told you that I was taking on this responsibility of your shower and I want to have the best day possible for you. But in order to do that, I'm going to have to leave Suzy out of the loop on some things, because she's just making it impossible. I just wanted to let you know. I don't expect or want you to get in the middle of it, but didn't want you to be surprised"
Or something along those lines.
Good luck.... weddings are supposed to be a happy, joyous occasions, but some schmucks have to go and ruin it with their selfishness.