“For instance, his grandfather George VI’s first name was Albert, but he reigned using one of his middle names. Charles could choose from any of his four names - Charles Philip Arthur George.”
If it’s Arthur, would it be the First, or the Second?
I’m not sure they count legends.
Like Pope Francis who was the first Francis, if Charles chooses Arthur, he would King Arthur, not King Arthur I. That only arises when there is a second Arthur, King Arthur II. Luckily, barring a cataclysmic event, we aren’t going to see a King Henry IX (Harry) or a King Archie.
I am guessing that Charles will reign as George VII. We will find out soon.
That’s a tricky question, but probably Arthur I because Arthur is a legendary king, not a historical one.