I have the answer! James's mother was not Sarah (Christian) West, but rather Sarah (Tayler) Knewstubb. She left her husband with her two youngest children, James and Amy, moved to Lyttelton, and lived with a James West. So my James was, I believe, born James Knewstubb but raised in the household of James West.
Lyttelton is where he met and married my GG-Mother, Emily Carson, so the story fits together well.
Now to find out why Sarah left her husband...
I'll also probably try to prove a DNA link to be absolutely sure. Thanks for the help!