I agree that 'William I the Conqueror' is quite wrong, and some punctuation, either commas or brackets, is needed to clarify the two styles.

Yep, me too. Not least because whatever the letters say we surely all read William I as "William the first", so there is an additionally unhandy the~the duplication. The only time I would actually say only the numeral would be a play by Shakepeare - but mayhap they do it diffurrinly in the US?