What files do I edit to be able to maximize Finder windows to fill the screen by shift-clicking the green button? Is there a group of files or a single file I can edit to facilitate maximizing "Finder" windows to fill the entire screen just as other application windows by shift-clicking the green plus button?
Try to use an application like Slate, Amethyst, ShiftIt, or Spectacle instead.

If you use Slate, for example adding

bind m:alt move screenOriginX;screenOriginY screenSizeX;screenSizeY


to ~/.slate makes option-m maximize a window.