I've been trying to remember or find where I got the idea that Macduff was son of Grimus [I]. It's an idea that makes sense to me because it would put Macduff far enough away from Macbeth's wife Bethoc that she might be heiress of the family without a strong contest from Macduff.
If Macduff was son of Giric [II], for example, he would have been Bethoc's 1st cousin. Close enough that he ought to have been "tanist" (designated heir), not her. This is a very subjective argument, I know, but it's hard for me to imagine that this powerful branch of the Fife royal family would easily acquiesce to an heiress married into the family of their rivals in Moray if they had a strong male leader who could contest her claim.
On the other hand, if Macduff was a son of Grimus, he would have been a 1st cousin Girich II and Boite. If he was a grandson of Grimus, same thing. He might have contended with Gruoch for the succession in Fife, lost, then thrown his support behind Malcolm III.
None of this would be anything more than speculation without some kind of evidence.
The other day I posted a reference to King Dubh taken from a genealogy published with the Welsh version of Nennius.
https://books.google.com/books?id=jdJXAAAAIAAJ&pg=PA284&dq=...
Did anyone but me notice the footnote about Grimus?
The editor identifies the Mac Duibh in the poem as Kenneth III. He notes "O'Flaherty says, 'Grimus, Scoticè Macduibh; hoc est Duffi seu Dubhondonis filius, quem proprio nomine Kenneth dictum invenio.'"
(Grimus, in Gaelic Macduff; this is the son of Duffus aka Dubhoda, whom I find properly named Kenneth.")
However, the editor admits there is confusion in this area. He seems to doubt that the brothers Kenneth and Grimus are really the same person but concludes that the Mac Duibh in the poem is probably Kenneth III.