The marriage record would only help, really, if it gave the names of the couple's parents or said where they came from.
It would be interesting to know why the people with online trees think that Angus' father's name was John - are they basing that on his son being named John Angus, or do they have some other piece of evidence? Because the fact that his first son was named Donald would suggest that his father was more likely to have been Donald.
I don't see where on the 1881 it says he spoke Gaelic, but if he did that might help to restrict the parts of Scotland he might have come from.
If he was originally Catholic, that too would reduce the areas he might be from, as most of Scotland was presbyterian apart from certain areas which remained Catholic - notably the southern parts of the Western Isles (Uist/Benbecula/Barra etc) and several pockets on the mainland.
One possible avenue of enquiry would be via Co Leis Thu?
https://www.hebridespeople.com - I believe they have some records of who emigrated from the Hebrides aboard which ship and when.
It might also be worth studying other families from Scotland who settled in the same place as your Angus. There could be clues among their families - for example children born in Scotland who emigrated with their parents and who can be traced to a particular place by their baptisms. If you were to find, for example, that all the Catholic Scots in the same part of PEI were from, say, South Uist, and that they all received land grants in PEI at the same time, it would be a reasonable line of enquiry to look for Angus there.
