Well, I’m still new to after effects so someone else can probably give a better answer… but couldn’t you just copy / duplicate your one layer that has all the masks multiple times and then just use one of the masks on one layer each?
About the adding motion graphics, I’m not sure exactly what you mean by this (is it 3D animation you’re adding in or something else)? If its 3D I can give you some good pointers on where to start but otherwise I am not sure… if you are concerned with the camera movements matching the motion graphics you are inserting, I believe that would be a “rotoscoping” issue