I have just read your posting regarding the New Zealand Mountcastles. I am a Mountcastle descendant on my mother’s side, her maiden name was Mountcastle. I still have some cousins with the surname.
There may be a small number in the UK but they are the originals!
I have a number of Mountcastle family history trees going back to the mid 1500’s for the UK. I seem to recollect having come across some information about branches that emigrated to Canada, Australia and New Zealand. I do not have the records to hand at the moment as I have lent them to a family member.
The major missing link is the family member(s) that emigrated to America. They may not even be on the trees that I have. Apologies to the American cousins I cannot help you.
Could your Christopher A Mountcastle be one of these on the following link?