Looks good. Your drawing skills have improved greatly too. I would have drawn one for you if I were smart enough to do it.
That is exactly what I had pictured in my mind, alternatively on either the ground or hot side of the additional relay from the ignition switch feed you could put a switch to be able to turn that light of if desired. I usually switch the ground side so that if a wire chafes and wears through to ground it just switches the relay on rather than blowing fuses. On that same note, fuse any circuit you use as close to the source as you can so the unfused section of wire is as short as possible. If there is a short in the wire before the fuse you will get smoke, so leave the possible exposure at a minimum.
I would think a 15 amp fuse would be good enough. 20 amp is getting into wire frying country with the gauge of wire we are dealing with in my opinion.
A ground is a ground and it all goes back to the negative terminal of the battery. As I stated earlier a redundant ground from the frame to the battery wouldn't hurt anything. ( I don't like to put all my eggs in one basket, so to speak)
A box behind the headlight if there is room for it is fine, just remember that the steering will flex any wires that turn with the fork so try to keep that to a minimum too. Mount the box stationary to the steering head if possible and have only the necessary wire flexing. Another possibility I would employ on my bike is that I would use the area where my battery used to reside for an electrical box that could hold a couple of relays and wiring needed. Though you would still have to run a minimum of three wires forward in that case.
Ask all the questions you want, I don't mind answering ones that I can. I know many others on the site feel the same way, so if I can't answer, or answer wrong, someone will set the record straight gladly. Much better to plan ahead and do it right the first time.