There seems to be an underlying assumption that the "Captain" who married the convict out of hand was the captain of the transport ship taking them to Van Dieman's land.

I have always assumed that the "captain" was the overseer of the convicts work gang. To me that makes more sense. If he was a ships captain, one effect of buying her freedom would be to enable him to take her home with him whereas "giving us good usage" implies, to me at least, that she remains on Van Dieman's land.

I read somewhere recently that it was not unknown for male convicts who were transported to be accompanied by their wives if they were married. In such cases once the convict had arrived at the other end his wife was put in charge of him - effectively he was in a form of slavery and she was his owner/overseer and was responsible for his behaviour during the time of his sentence.