Hey, have you checked the color of the carpet, looks burgundy to me! Hard to tell in the picture, but it might be! I'd leave the chairs uncovered, it is a big expense for something like chair covers, whether they are homemade or rented. And since the carpet might be burgundy, at least the chairs match that. I'd pick a color from your bridal pallet and turn MIL loose on the bows for the chairs, if you do anything at all. Mix it up with the aqua and the cream, or just stick with the cream.
Think of it this way, people will be sitting in the chairs, they won't be craning necks looking at the burgundy, and once they are all seated, you can't see the upholstery on the chairs anyway. And if you put a great Italian feast in front of them, nobody will be looking at anything except their plates. And you of course.
And if anybody has the nerve to say something about the colors in the room, and why did you pick the place with the burgundy when you have aqua, just ask them, "did you like the meal? and when they say yes, say "that's why, the food is good here."