I know that William was a member as he is buried at Centre Friends Meeting House Cemetery in Randolph County, NC.His father, David, married Mary Trotter and he probably was the first Quaker but I can't prove it.
I have no idea the significance of Dunbar castle to the Coltrane's.
We just had the 68th Coltrane Reunion.It is held the third Sunday of October at Centre Friends Meeting every year.It is a covered dish affair and all are welcome.We really need some people to get involved to keep it from dying.Interested parties can send their name and mailing address to Bettie Coltrane.Her address is:firstname.lastname@example.org or: Bettie Coltrane 200 Merrimont Dr Winston-Salem, NC 27106
She is Secretary of the Reuion and will send you a reminder.
For those planning to travel to Wigtown a stop a GC Books would be well worth your time.They are nice friendly people full of information on the local history.The owners are Mr & Mrs John Carter.Tele:011-44-01988-402499.