In a previous message you mentioned the possibility of William's death connected with the de Hautevilles. Have you found new information, or was there a conspiracy between Arnulf and de Hautevilles?
In "Historical Atlas of the Vikings" by John Haywood, it says, "Rollo was made count of Rouen. (the rulers of Normandy did not use the title "duke" before 1006)."
I found in "William the Conqueror" by David C. Douglas the description of how through the reign of the Counts of Normandy they bestowed upon their many relatives the title of Count also. Finally it seemed that Richard II began to use the title of Duke to distinguish himself above his relatives and other counts. I think there have been references to the title of Duke being used much earlier in rare instances (and these might be a corruption that crept in from more modern authors) than 1006 and William the Conqueror used Count occasionally much later than 1050. We may say that generally it is more correct to use Count when speaking of Rollo, William his son, and Richard I and then with Richard II, III, Robert and William the Bastard/ Conqueror it is more correct to use Duke.