As my sister says, my mother is not a "taster". Everything my mother eats must be liberally doused with salt and pepper before she will try it. DFIL is a good cook, but very absent-minded and will wander away from whatever he is supposed to be minding, so we do the vast majority of the cooking for family meals. MIL is of the opinion that no vegetable is edible until it is boiled to mush and doused with margarine. MIL doesn't eat the following: chocolate, cilantro, blueberries, cranberries, cherries, avocado, spinach, raisins, blue cheese, and any dressing other than ranch. We usually leave some dishes without any of the above, and add to our own to taste.
When I am cooking for a crowd, I cook everything to a safe internal temperature and season moderately. I provide sea salt, a pepper grinder and hot sauce for those that like to have more of whatever seasoning. When cooking for more than 6 people, including our three, I make two different mains so I can please everyone. We have several friends that don't eat red meat, or bone-in chicken, or boneless chicken, so it becomes difficult to accommodate with one main. For example, I will make a taco/nacho/burrito bar with both grilled fajita chicken and skirt steak and all of the toppings. Vegetarians are happy with the cheese, beans and rice.
I don't think you should have done anything different with your MIL.