I do know something about the people you mention. Angus Campbell was son of Roderick Campbell and Marion [?] MacCormick. His wife was also a Campbell by birth, daughter of Ewen Campbell and her mother was a Steele. Angus and his wife later lived in Geirinish and he died in Lochcarnan on the north coast of South Uist. His sons Roderick, Angus and Ewen all settled in Benbecula in the eastern pendiles, Uiskevagh and Kilerivagh just across the South Ford from their father. There are descendants still in the islands.
As to Donald MacAulay, I have his place of birth as Kildonan rather than Hacklet - or did I see another posting where you mentioned Lionacleit? I would be interested to know the source of that info. as I am not aware of any Census or other evidence to place John or Mary Catherine in Benbecula rather than South Uist. The marriage 'at Iochdar' will of course just have been the recording of the marriage at St Michael's, Ardkenneth as conventionally Mr Seumas [Fr James MacGregor] married the couple in the relevant township, in this case I assume probably Geirinish.