As a Roman Catholic, Mary would not have been summoned by the Kirk Session, but Archibald could have been, if he was a member of the Church of Scotland. If so, and the KS minutes have survived, that might confirm that he was the father.
I still think that young Archibald's parochial board application, if it can be found, is likely to be the key to this.
If Mary had been in residence in Kilmory for 7 years by 1866, she must have been sent out to work there very soon after the death of Alice Corrigan or Norris, which further strengthens the idea that Patrick and Alice were her parents.
There is no statutory record of a marriage of any Mary Norris between 1867 and 1873, to William McLaughlan or anyone else. Nor is there a reference in the RC records, or in the Irish statutory records, or in FreeBMD.