I've just come across this posting and hope you still look for any follow-ups.
I believe the John Thorburn who went to Swaziland and was the father of Alice M THORBURN, was actually born in London in 1838; I have more information about him and many more family connections. He was my second cousin 4 times removed so, if you're descended from his daughter, we are also related, probably 6th cousins.
Let me know if this is of interest and we could exchange information. Regards, John Brown