This almost sounds as if you are being tested.
How many people could just make 6000 cookies?
I would just say, 'Sorry but I have worked out the logistics of that and I am not going to be able to do that particular task. Maybe we could have a meeting to discuss the jobs needed and we will work out what we are able to offer. Incidentally, in British culture it is usual for the parents of the bride to pay for the wedding but we are very happy to contribute and we will let you know how much when we have more details of the wedding plans'.
You have to stand up to bullying when it starts whoever is doing it. Is your son aware of any of this? It will be hard to bring him in because his natural instinct will be to protect his fiancé but maybe you could say to him 'sorry I'm not able to help with the cookies but I'm waiting to hear from X in what other ways I can help'. If you keep quiet she will be painting you as unhelpful and he won't know any better.