The description of the "Blood of Cerdic" line seems highly irregular. I am not an expert on the practices of early Anglo-Saxon kings, but AFAIK they accepted Christianity sometime around 650 or so. Aethelred the Unready did not personally murder his brother, Edward the Martyr, since he was ten years old at the time. He also seems to have been Christian, like most other contemporary Saxon rulers. If the Wessex kings had an oath demanding that they drive the Christian kings into the sea, it stands to reason that it must have disappeared when they accepted Christianity themselves.