While I have very little to add to your Martin/Leitch families of Glassary, I will throw this tidbit out there: Margaret Leitch married Alexander McTavish circa 1790 in Glassary (earliest child, Hugh McTavish born 1790). Their son, Lachlan MacTavish, was a member of the Kilmichael Glassary Church of Scotland.
https://findagrave.com/cgi-bin/fg.cgi?page=cr&GSln=mctavish&GSbyrel=all&GSdyrel=all&GScntry=50&GSob=n&CRid=2576305&df=all&pt=Kilmichael%20Glassary%20Parish%20Churchyard&I was able to find her approximate year of birth from the 1841 census when she is living with her son Hugh McTavish. I believe she is buried in the Kilmory old graveyard but have yet to confirm the location (Kilmory estate or ?). Margaret MacTavish, 77, 1847 which roughly corresponds with the 1841 census dob of 1771. Since she died pre civil death registration, my trail has a roadblock.
Point of all this is John Leitch would be her contemporary ... brother? cousin?
As to the Grahams, there was a John Graham of Dougalston who married a Margaret Campbell heiress of Archibald Campbell of Shirvan in 1747 and he legally changed his name to Graham Campbell. This Graham Campbell family was living at Castleton, Argyll.
The Kilmichael Glassary records have gaps. Some may be due to lack of funds as there was a fee for baptismals (Lachlan was on a statutory labour list for the year of one of his children's births). Some may be due to lack of clergy as there are gaps between ministers. Some may be due to loss of registers. Kilmichael, Kilmartin, Ford churches have now been merged.
http://argyllpresbytery.org.uk/parishes/An Archibald (age 75) and wife? Margaret (age 65) Leitch family in the 1841 census at Balimore that was captured with my extended MacTavish family search.