I never really got into it, though I did finish reading it, because I disagreed with her so much. It's not my period, but I got interested in Macbeth after a holiday spent in Lumphanan, and again when I investigated various forms of dual kingship in Europe. The one in Scotland was alternating lines, so Macbeth was the rightful heir and Duncan the usurper. Macbeth had an incredibly long reign for the time. And place, and managed something almost impossible: he went on pilgrimage to Rome, and was still king when he got back. I'm inclined to suspect that the reason Shakespeare's play has its reputation is because Macbeth's ghost is so furious at being traduced.

Suggesting that two men were the same because they never appear at once is interesting, but one might as well (or better) suggest that Aelle, King of the Saxons, was King Arthur.