Thought for a while about your note.. Family came to Elgin in 1826-7 from Glasgow. The Morison's had been in Glasgow at least for the previous 100 years and very sure were not highlanders or ever had been. A family estate east of Glasgow called Craigend was sold in the late 1870s.
As to one or two 'r's think likely the generation in the 1860s normalized the spelling. But there may have been another reason. The father of the William Morison arriving in Canada in the 1820s had died due alcoholism and drugs around 1820 and afterwards the mother REMARRIED. To the socially repressed Canadian Scots this was extremely embarrassing and maybe the spelling change helped get some distance.