I have finally found out why there was so much interest in my maternal Grandfather's genealogy. He matched the MacIaian's of Glencoe.
http://scottishancestry.blogspot.co....ald-chief.html
There are only a handful of people who match this branch of the MacDonald family and my grandfather happens to be one. We have found evidence of the name change from MacMillion to Williams but not the MacDonald to the MacMillion.
The only reference I have found of this spelling is in a poem which refers to a Rich Man who is buying up sheep to put in the Highlands.
This will remain a mystery because of the lack of a paper trail.