I don't have any hard facts for you but I was looking in my fathers books, "Our Calhoun Family, Volume 1 and I may have found the Alexander that you are researching. First of all, when the Colquhouns migrated from Scotland to Ireland, the name changed to Colhoun and that may have caused your research to lose track of Alexander. On page 510 of Volume 1, there is a son of Galbraith Colhoun & Annabelle Clements of Sixmilecross in Tyrone, Ireland, who was named Alexander, born approximately about 1812. He was living in Mountfield, County of Tyrone and there were so many Colhouns living there that my father could not sort out how they were connected. Alexander was one of 9 sons and 6 daughters and several of them migrated to New Brunswick and Ontario.
I hope that this information will put you back on track in your research.