Glad it was useful.
The marriages mention in the posts here, I don't think relate to this Henry Mitchell. The newspaper reports indicate that he was living in boarding houses or in gaol and no mention of other wives, plus it looks as if he could be the Henry Mitchell who died in 1886.
There are mentions of a baby born on the voyage and/or that Sarah died in childbirth but on the Zealandia records it says she died of chronic bronchitis and diarrhoea and no mention of a child. The newspapers mention 5 children which are the older ones we know about. There are too many William Mitchell's to work out which one he might be, but dates that would match could be 1925, 1940, 1945, 1951 if that helps. Given that the mother and stepmother Elizabeth were off the scene and Henry patently couldn't give a toss for his children I very much doubt the children came back into a family situation with him. They most likely were licensed out as farmhands or servants as soon as they were 15 under the Industrial Schools Act and William may have made a life in the Canterbury Otago area as he nothing to back to Auckland for.