The only thing in your list that actually happens is the Thanksgiving Day and Christmas Day special meals. There is no other seasonal pricing to buffets and AYCTE meals.
The only other time prices are different are when they are in conjunction with an event package such as F! dining package, ROL dining package, Candlelight Processional dining package. ALL of these have the prices clearly laid out even on Disney's website.
There is no apparent reason why Disney doesn't put the actual buffet price on the menu.
With seasonal menu planning, the cost factor plays into the seasonal food, things like fruit and veggie's that are not being locally grown due to the season here in the states, and has to be brought in from other countries. Everyone wants a variety of fruit, such as berries, they are not grown in the US year around, so they have to come from somewhere as guest expect them.... Normally on a buffet, there is always chicken, beef, pork and some type of seafood whether its fish, shrimp, or a variety of option like in a seafood alfredo pasta dish... If there has been a issue with livestock then they have to look else where to get the products.. Whether it is at the farm and ranch level, natural disasters, such as flooding, drought, disease, whole crops can be wiped out in a couple of hours... or recalls due to tainted food during processing... they still have to fill the buffet, as guest expect a certain level of service.... so it just seems that the price would fluctuate...
As well the price points cover all three meal times ( I think) , breakfast at the buffet would hit the lower end, lunch hitting somewhere in the middle and dinner on the higher end..
Trying to hit a little something for everyone and create new and interesting menu offerings is a huge task and then add in all the different diets lifestyle then include lots of people like me that has food allergy.... to me seems like a daunting task. So I'm okay with the price point spread...
If you are budgeting, then just go with the higher cost for practical purpose's, then if you pay less then you will be happily surprised...