Greg - A few questions.

1. Have you seen the original entry of marriage, because the full certificates with father's name came into use during 1849.
2. We might get a clue to Margaret's origins if we know the names given to the children.
3. As to Fletcher Christian, most emigrant Manx families of Christian claim relationship to him, just as all the Cains claim Hall Cain, all the Quilliams claim Captain John Q. & so on.
There will be a good deal of research to do before we can tell if there is a relationship with Fletcher Christian.