I think the above rules are going to be hilarious in the very near future, considering we're now squarely in the teen years for kids who were never taught cursive and will often have to print their names or scribble some nonsense. And before anyone says "oh but my kid/s learned" - I totally get that some districts likely still teach it. Somewhere. We moved from the east coast to the west coast in the elementary years, and it was long gone from both my now-teens' curricula in both places. (My eldest only learned b/c she was amenable to my teaching. DS15 - um, nope. And making him write it like once a year when needed has not produced a repeatable version.)
OP - My son would *absolutely* put his nickname and need to be told to add the rest on, so I feel your pain. Personally, I'd just have my son add on to his name as best as he can. There is almost no way that whatever my son puts on his passport will look like his next signature attempt anyway.