I am checking back in and have read all of the responses. I will add some information for clarification:
There are no food allergies except for the one brother who can't have caffeine; since there is caffeine in chocolate, he doesn't eat chocolate.
We always check with the hosts about side dishes so that there won't be duplication. We are simply disagreeing on what apps to bring.
My in-laws are not adventurous eaters at all - except my FIL. This is why DH doesn't like to try to introduce anything new. I do not feel that stuffed mushroom caps are so exotic that they won't try them.
DH is the cook and we will not be allowed in the door if he does not show up with our strawberry soup (recipe obtained from WDW 1900 Park Faire). The soup consists of heavy cream, Cool Whip (sugar), frozen strawberries (sugar), and a couple of cups of... sugar.
His other popular annual offering is the little sausages, wrapped in bacon, and then covered in brown sugar before baking.
We also usually bring a hot dip that you dish out with bread or crackers and at least one other person brings another hot app. I usually put a token cracker or two on my plate and then eat the dip with a fork and then toss the crackers. Trust me when I say that hot dip without the crackers is a very sad experience but this is my life so I do it.
My feeling is that with so many bringing something, there will be no one forlornly looking at the food and thinking that there is nothing to nibble on pre-dinner - except me, that is. This is why I am telling DH that we should bring something that I can eat too and maybe they will love them and they will become a regularly demanded item and maybe we will bring them home as leftovers. Either way, there will be plenty of options for everyone.