.......What the difference in taste from just using 1 tablespoon flavoring to your mixture? .........
As I have said, flavoring is a very personal decision.... What I like and what you or your family like can be very different. (Example: Do you LOVE snails or rattlesnake meat; highly spicy foods, etc?? I sure don't)
I find one Tblsp flavoring in this recipe to be just right....not overpowering but it brings out the great taste combination of all the ingredients.
Adding oil and puddings increase your costs....to me they are not necessary.......but again it goes to personal taste likes and dislikes.
Using fruit juices (&/or fzn concentrates such as lemonade) will increase the flavor of such fruit based cake mixes.
In a chocolate cake use strong brewed coffee as the liquid ....it really boosts the chocolate taste

(This is from someone who **HATES!!!!!** coffee)

....... what are the differences in using buttermilk/milk vs. water........
Buttermilk could throw off the science of the ingredient combination.....I don't use it.
Milk or cream vs water - many think it makes a richer taste.