My god, ethelb , I could have written your post myself and I have used the Phoebe Buffet Cups/Ice analogy many times whilst trying to see the funny side of it. I love my MIL dearly, she is a kind, generous and funny woman and I am very very lucky and I do appreciate that she is just excited, but something about weddings just makes usually sane people lose their shit.
My MILs other DIL kept her at arms length throughout wedding planning, and I thought that it was quite unfair of her given that MIL only has sons, so when DP and I got engaged I made a point of trying to actively include her. I should have cottoned on to why DSisIL would do that when I mentioned that DP wanted a thistle in his buttonhole, and MIL then came back with a suggestion of adding a rose and a leek to it to cover all of Great Britain and that she could get him some tartan trousers to go with "the theme". Weeks later after various other similar incidents, mere hours after DP had told her she needed to calm down and back off a bit, she sent me an email (actually, 5 emails within 12 minutes outlining her reasons) suggesting that we stop beating around the bush and just appoint her The Wedding Planner officially.
She was clearly disappointed at being left out of dress shopping as well, as we turned up at their house one day and she started showing me pictures of my dress on her ipad and how she thought it could be bustled (right in front of DP!), having somehow pieced together exactly which dress it was and where it was bought from various conversations that we had had. I am not really a 'wedding' kind of girl but I really was looking forward to the big reveal-as-you-walk-down-the-aisle moment, so that was a little disappointing for me.
OP, ignore the screeching MIL-defence brigade and again, I echo ethelb in telling you to ignore them. Organising a wedding is stressful enough without having to deal with people-managing and politics as well. Good luck to the both of you with the wedding planning!