Spider gene has nothing to do with this. Spiders actually are often the best eaters and any wobble does not interfere with their feeding so don't consider the gene a culprit. Age plays no factor in it either. I have an over 7 year old spider that has no feeding issues for example.
How are you measuring your temps? What are you using for heat and how are you controlling it (as in thermostat)? High 80s and low 70s seem off to me.
Ball pythons can not get "red" they lack the ability to get flushed so unless he has consumed a large amount of substrate or his glottis (little breathing tube that sticks out when they swallow a prey) is blocked, they are almost incapable of choking on a meal.
You just switched feed sizes but you do not know the weight of your snake. How are you heating and defrosting this prey? How fast between downing one mouse are you offering the second?
How warm is the prey when you offer it or are you offering live prey? How do you present it to the snake? Meaning how is it given... on tongs or tweezers or just tossed in the cage?
What else have you changed? Anything you can think of from decor to substrate to the heating/cooling of the house.