If she contacts you again I think you should say "I don't know what you want from us. We tried to plan a beautiful themed shower for you, all you had to do was give us a guest list with addresses and show up. Instead you told us that you wanted to plan it yourself so we backed off. Now you're upset that the shower isn't nice enough? I'm sorry but it's too late for us to change your shower."
POD to this. I do think there's a lot of drama and emotions flowing around from everyone, and a lot of mixed messages. From this and the wedding behavior, I get the sense that Amy likes drama, and she likes having things both ways. It's almost like she sabotaged your attempt at throwing her a nice shower, just so she could lament you guys not throwing her a nice shower--like it's suddenly totally out of her control, when the problems started because she
wouldn't give you the basic info you needed and she
insisted on her sister being in charge (changing the offer you'd made).
With the wedding, I see something similar. She ends up excluding all
her BFFs from her wedding party because of actions started by one
of them (Cynthia--whether she
was out of line or not, I can't say), and then spends her time lamenting that you guys weren't in the wedding party. Well, whose fault is that? I get the sense she acts like a meteor falling out of the sky is causing these problems, when in fact it's her own actions. And there's nothing you guys can do to make her "happy" with these situations, because apparently, it makes her happy to complain about stuff that didn't turn out the way she "wanted."
Completely just my own observation from what's been posted here, of course.