Do you have to cook corn tortillas before you put them in a casserole?!
They will be fine without pre-cooking. They are already cooked on a griddle before packaging, otherwise they would not hold up. Sprouted corn tortillas can get a bit mushy. White corn tortillas are best for casseroles. They don't overpower anything with the corn flavor like yellow corn can. Like one other answer, be sure to cover them with sauce/cheese before baking so they don't dry out. Also, corn tortillas are easier to cut if you put them in whole like a lasagna type of dish, flour tortillas can be hard to cut if your dish is a bit on the dry side.
I like to use crushed corn chips for the top of a casserole with Mexi type seasonings as well. But I also have used potato chips crushed up for a "topping" on a casserole, added before baking. But usually ones with other flavors than south of the border style. For those, corn chips are best or the corn tortilla that has been sauced and cheesed. A few dollops of sour cream is nice and when it is finished baking, some chopped fresh tomato and fresh chopped cilantro is a nice touch. Not too much cilantro, it can get overpowering or soapy in flavor for those trying it for the first time or for those that don't care for it's flavor. Use fresh cilantro like you would parsley.
No, you would be better off not to. They will cook quickly in a casserole. Just make sure they are covered with whatever else will be in the casserole so they don't get crispy.
If they are raw corn meal then yes or they will just turn to mush. If they are cooked and cooled "raw" un-fried tortillas they will work fine right out of the bag.