Yeah, I totally agree with everyone else - I don't think it is a good idea. You said it isn't just a money issue but also things like expectations, schedules, etc...I would honestly hope that I would be able to trust someone in my wedding party enough for them to understand how important my wedding day was to me and because of our friendship I would expect them to help me any way they could, not by a contract. IMO friendships come from trust, communication and all that stuff, you don't create the benefits of a friendships with a contract.
I think you might be stressing about this one, pressured and worried . That is all totally understandable. You have other options and I think better ones. Let you BM know how much you are worried about this, lay it all out there and be honest with them. Chat about it and I am sure you will feel much better. If need be then don't have a wedding party, I didn't and I love not having to worry about that during my planning and the day:)