I agree with you that you should tell your mom that the shrimp feed sounds great, but to tell dad not to make too much as unfortunately, you are the only one who likes cold shrimp so will be the only one from your family eating them. It would be a shame for him to spend a lot of money on shrimp for them to be not eaten and then thrown away.
Once you have told her, it is out of your hands. If he still insists on making them, that is his choice. However, your family does not have to eat them. I would not make the boys eat any if they don't want to, nor your dh. If they are too salty for you and since you must watch sodium intake, don't eat them just to pacify your dad. If he starts to bully the boys, stop it immediately "Dad, the boys do not like this type of shrimp so they do not have to eat them." "Dad, I am under doctor's orders to watch my salt intake. Unfortunately, these are too salty and I am unable to eat them."
If he chooses to pout and hide, that is on him, not you. He is a grown man and you cannot control his actions. As long as you are polite in your interactions, there really isn't more that you can do.