Most of the good stuff's already been said, but I'll add this: how the fiance handles his parents' manipulation now is a litmus test for how well he can hold them off over the years. If he caves in, then I'm afraid he wouldn't be a good match--however, I think you've said the couple is going ahead with everything, and lovingly telling the parents "I hope you can make it, but if not, we'll miss you," and if that's the case, then the fiance is probably the type to stay strong over the years & put his marriage first, & not buckle under the thumb of the parents who want a grown son to do things their way.
I'm sorry this is happening; what a tough situation.
If you're afraid of butter, use cream. ~~ Julia Child
As you cook, you enjoy omniscience about food that no amount of label reading can match. Having retaken control of the meal from the food scientists, you know exactly what is in it. (Unless you start w/cream of mushroom soup, in which case all bets are off.) To reclaim control over one's food, to take it back from industry & science, is no small thing; indeed, in our time, cooking from scratch qualifies as subversive. ~~ Michael Pollan