Pre-made meals are tough in this aspect, but it can be navigated. What the manufacturer does is takes the nutritional value of all the food they are giving you and then dividing it by the number of servings they see fit. A lot of times they will give the serving size in grams or ounces. So if a serving size is 10 oz, this means 5 ounces should be made up of your meatloaf and 5 ounce from your mashed potatoes. The easiest way to look at it is if you put it all into a blender and mixed it up, thats how your measurements would be done.
The other part of your question is, in reality, there isn't really a large difference between pre-cooked and post cooked meats. Now, for lean meats (turkey, chicken, etc) there is no little to absolutely no difference. It is only in the meats that are higher in fat that you see a difference, aka beef and pork. But the change is still pretty small and not sure I would worry about it. If anything, it will push your macros more into the fatty area and you can adjust by eating less fats throughout the day to hit your goal