OwenM said:
(if it was post-Norman Conquest, William would likely be William II, as he was of Normandy).
Nope. William the Conqueror is William I of England for it is he who conquered England. He can't be William II post-conquest when he is the one who did the conquest in the first place...
Unless you mean he kept his regnal number as Duke of Normandy. Thing is the numbering is split between two realms. So William the Conqueror was King William I of England
and Duke William II of Normandy.
A more convenient example of this are the Spanish Hapsburgs and the huge number of titles they held. Holy Roman Emperor Charles V was also King Charles I of Spain, Archduke Charles I of Austria, Duke Charles II of Burgundy (titularly), Duke Charles II of Brabant, Duke Charles III of Luxemburg, Count Charles III of Flanders, King Charles IV of Naples and King Charles II of Sicily among many others.
His son King Philip II of Spain was Duke Philip V of Burgundy (Titularly), Duke Philip IV of Brabant, Duke Philip III of Luxemburg, Count Philip V of Flanders, King Philip II of Naples and King Philip I of Sicily as well as King Philip I of Portugal and King Philip I of England
jure uxoris (though the latter is contested).
OwenM said:
If he wants to be Arthur II, he'll be Arthur II, consistency be damned (or possibly introduced)
I think that, on the contrary, Chroniclers love consistency very much. Which is why the English monarchs are numbered post-conquest because otherwise, we would be calling Edward Longshanks Edward IV instead of Edward I. Not to mention that Henry VIII's son was named Edward VI, and you mentionned Henry VIII as the first to use a consistent regnal name: which would mean that Arthur would most likely follow that path and be Arthur, first of his name, King of England by the grace of God.
OwenM said:
It wouldn't exactly be the first time mythical monarchs have made it into the regnal numbers - look at the Popes, or the Kings of Sweden.
The Regnal Numbers of the Papacy are a mess true but that's because the numbering acknowledge some Antipopes (that shouldn't count): so it's not because it uses fictionnal people, but because it uses people who were in conflict for the Papal Tiara with the official Pope. The few Popes that could have been fictionnal are probably among the first ones and even then that doesn't do much in terms of regnal numbering because the tradition of taking a Papal Regnal Name came long after the Papacy came to exist when Mercurius became Pope John II in 533. Sure you had a few popes who kept their first name as regnal name (the last one was Marcellus II in 1555) but the regnal naming tradition is a pretty solid one.
Sweden is a bit more relevant, but then again I think this only applies to the Charles and Eric who ruled over Sweden because of a few acknowledged mythical figures. Otherwise, the naming remains pretty consistent.
I think overall we can point out that this basically means that we have a regnal numbering error that was kept for consistency in both cases... People don't really care that Charles XVI of Sweden shouldn't be Charles XVI after all...