You shouldn't confuse legal name and nobility titles.

I don't, but many, many, many people entering names and titles in database systems from previous jobs did. And not just nobility titles, but also acacdemic titles, job descriptions ("Accessor des Bergfachs" is my favorite one - I have no clue what that job is, except that it has something to do with mining). I was really happy not to find "cook" and "gardener" as titles ...

I'm pretty sure I read once, that he signed his marriage papers with Diana simply as "Charles Mountbatten-Windsor".

Sounds plausible. The fully "decorated" name with all titles and middle names would probably fill half a page and take half an hour to write down.

Alexander