That WASC cake sounds amazing! I bet that would make awesome cupcakes with a nice dense, moist crumb...make something small in size feel really rich and decadent 

I'm aiming for the whole light-and-fluffy aspect mainly because of frosting: my mom always mentions this incredible marshmallow frosting my grandmother would make (which from what I can tell, looks like 7-Minute Frosting made with both light corn syrup and cream of tartar, with 16 regular marshmallows chopped up and whipped in). She says it was a very lightweight frosting on vanilla scratch cake; so I know I need to make a "lightweight" cake layer if I don't want it to come compact--or squish out all the frosting from the middle. 

(I think it's also kind of a Mount Everest thing with me--I want to know I'm capable of making a moist yet fluffy layer just because I can )

A plain old white cake mix is light and fluffy, but it will not taste nearly as good as the WASC recipe or a scratch cake.

I've never made frosting with actual marshmallows in it, but I've made a lot of frosting similar to 7-minute frosting and it is not as easy as it looks.  I now make Swiss Meringue Buttercream, with just a small portion of the butter, if I want that type frosting.  More dependable.