For anyone who might read this, FamilyTree has a second wife of James.
Namely, Sarah Ann Warren, the theory goes that they married on 10 June 1845 in Launceston, Tas.
I haven't included this in the tree as I can't find a solid source for it.
There were records for one (highly suspect different) James Mills marrying a Sarah Danner on 10 June 1845 with James being 30yo and Sarah 18yo
Seen here: https://stors.tas.gov.au/NI/833234
I couldn't find any record on the demise of the first wife, Mary Ann Mills nee Bailey either, though it is possible she matches the Mary Ann Mills accompanied by 2 children leaving Tasmania for Victoria seen here: https://stors.tas.gov.au/NI/585914
Sarah Ann Warren and Sarah Daner are the same person. She was the illegitimate daughter of a convict and taken into care because her mother could/would not. She changed her surname to Daner before she married James Mills and they subsequently had a large family. James Mills and Mary Ann Mills' two surviving children (a boy child died in childbirth and he was not named) moved to Victoria in the early 1860's and lived together in Collingwood for a while before Sarah Matilda died of Tuberculosis leaving an infant son that carried the name of Cox. They are both interred at Melbourne General Cemetery; Sarah Matilda in an unmarked grave.