Somebody has forced another marriage on Ethelred, on the bizarre assumption that he HAD to be married to "somebody" in the MacBeth lineage (???WHY???), that he HAD to be the progenitor of the MacDuffs of Fife, that he HAD to be the same person as Constantine mac meic Dubh (which is flatly *crazy*),and all kinds of yadayadayada.
I thought this question had been settled two years ago. Guess not. Guess it will NEVER be settled, because people just HAVE to have their fictitious lineages. :-P