I think it would be hard to sit back and do nothing, especially if you know that it is happening. I am sure that your mom will do the best she can, and even if it doesn't end up being what you would have done, seeing how happy your parents are about it will be worth it.
I am sure that it was hard for them not being able to be there for the DW, so this is their way of being a part of your special day. I am sure that it will still look great, even if it isn't what you had imagined.
You need to find a way to relax and get your mind off of it! I'm sure it will all work out great.