Pat - I have read with much interest the various messages about the McGachen family. I am a relative of John Drummond McG and his 2nd daughter, Sophia Euphemia Auld McG, and have quite a lot of information about that side of the family, including the family bibles of the parents of Sophia and Mary Dobson (they were sisters) and of John and Sophia McG. Both give names and dates of birth of the children. There were 7 Dobson children but Sophia and Mary were the only ones to survive into adulthood. John and Sophia McG also had 7 children, but 3 boys died young - Francis Nicholas Styleman, Stewart John Drummond and Duncan George Matthias. The other 4 children, Frederick William Dobson, Henrietta Mary Armine, Archibald Hew and Sophia Euphemia Auld, all married and have living relatives (although, as far as I know, none still have the McGachen surname).
Please tell me if you want more information. I don't think I have anything to add to what you already know about John McG and Ann Dew. I believe that their son Frederic Stewart McG emigrated to Canada so there is probably quite a lot to add to that side of the story.
Best wishes, Patrick.
PS I am new to RootsChat and so just hope you get this message!