I apologize for coming late to the discussion, and don't know whether this is a topic you still are interested in. I'm not a genealogist, that role is my mother's, and it is she who would be able to provide the most information. She has done a considerable amount of research into William's background, and as far as I know is able to document much of it appropriately
William Harry Trehaeven is/was my grandfather.
Before I sketch in broad strokes my own understanding of William's story let me say that my mother is unfortunately almost completely computer-illiterate, and does not currently have access to a functioning computer. To complicate matters, she lives in Port Elizabeth, South Africa, and I live in Denver, Colorado. My brother, who lives in Port Elizabeth, has access to e-mail and could serve as a conduit for questions posed to my mother, but most likely my mother would prefer to correspond via snail mail. If you are interested though I would be happy to explore these, and other possible options with you.
My grandfather died in 1965 in Kimberley, South Africa. At the time we, his family, believed he was of Cornish origin, who came to South Africa after the 1st World War. My aunt Julia, William's eldest surviving son, Desmond's wife, had tried to trace information about William and the Trehaeven family on holiday visits to the UK after my grandfather's death, but met with no success.
Then in the mid 1970s my cousin, Robert, Julia and Desmond's son, met a Reverend Ken Terhoeven when the Reverend came to Robert's school to give an address, saw Robert's name on an honors board and asked to speak to him. Reverend Terhoeven told Robert that our grandfather had been born a Terhoeven, and that during the 1st World War had changed his name to Trehaeven, and had cut off all contact with his (Terhoeven) family, even after returning to South Africa. The Terhoeven's, though not large family had deep South African roots, the first Terhoeven having come to South Africa in the last quarter of the 18th century.
As I said my mother has done a lot of research and has proved to her satisfaction that the Reverend Terhoeven's story is correct.
As far as I know the mechanisms by which my grandfather changed his name are not known, and it's more than possible it was not done officially. We don't know if my grandmother knew anything about my grandfather's background, it's possible she too was told and accepted the Cornish origin story, and that she only met and married him after he'd effected the change. My mother's father, James Garfield Richards, was in fact a member of the Cornish exodus to South Africa, and there was never to my knowledge any hint on his side that he doubted William's origin.
I am in regular contact with my mother by phone, though with the time differential I usually limit my calls to weekends. I'd be happy to explore contact options with her, should you wish to contact me.
Regards
Robin