I think WBCCI folks tell you to use a hair dryer to heat up decals which will allow them to peel off. There are other products, one I know called Undo which allow the sticky gunk to soften.
The ghosting is a different matter since what you have is a finish that has been protected from the UV and weather while the rest of the area has not been. I'm really not sure if there is a good way without involving something with an abraisive. I think most folks just live with the ghost.....or join WBCCI and see if the ghosted numbers are available. This is sort of why I didn't put mine up when I belonged.......now that I dropped out, I'm especially happy they weren't up there.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700