Okay, the egg question has been answered with what I was going to say, so I won't answer it again!
As for the shortening/oil/butter discussion: all I can say is that a butter cake and a cake made with shortening/oil are two completely different kinds of cakes. I generally prefer butter cake unless I am making a sponge cake, so I don't use shortening in my cake (I used to use half shortening and half butter in my RV cake, but since I switched to all butter it is more moist). The only cake I use oil in is my carrot cake, but then, as Alton Brown said, carrot cake batter is basically a quick bread/muffin batter. The only cakes that I make that use melted butter are genoise, which require clarified butter. All of my butter cakes use room temp butter.
As for adding pudding mix, unless you are sure that everyone who will eat the cake is NOT allergic to aspartame, you should NEVER use the sugar-free mixes in any cake.
When it comes to margarine, it is not allowed in my house. Margarine so chemically close to plastic it's scary. Additionally, it is not regulated by the FDA, because it is not considered food; it is a manufactured product.
I do use some shortening in my cooking, but I try to limit it as much as possible. Even in biscuits and pie crust, I have returned to the tradional lard and butter.
I've found that when it comes to baking and cooking, the higher quality and more natural the ingredients, the better the outcome, every time.
Sorry this post is so random.