Does anyone know if it is possible to apply a keyword to a folder and then have each index card within that folder automatically have the same keyword upon creation? Any help would be greatly appreciated! Thank you.
No, that would involve far too much guesswork and presumption from the application. Consider if you drag a keyword onto a top-level folder, say Part I, that says “Fix typesetting of title page”, and suddenly that keyword gets plastered to 287 documents, all the way down to paragraphs that were split off for careful editing. Not cool. 
And really, there is no way Scrivener could guess at just how uncool that would be. What make sense to you with one drag is a nightmare for another with some other drag.
What you can do, however, is select many items at once in the outliner or binder, at many depths, and drag a keyword from the HUD to that selection. They will all acquire the keyword simultaneously. This leaves the decision process of what gets a keyword to the system that can best make that decision, you, and gives your brain an easy way of doing so without fifty tedious drag and drops.