... anyway, the following is just "my take" on what may have happened.

[Now that we have some new info to hand. ]
* That during her time in Molesworth Street and the troubles she faced (poor woman, it must have been one hell of struggle for her as a widow), Mary Ann MASON had some of her children removed from her care and sent off to live with others.
* Marshall John MASON (later Donnelly) is raised (probably from at least 1877) by (Sergeant) James DONNELLY and his (real) wife, (also) named Mary Ann.
[I think you were right earlier when you said how unlikely it seemed that James DONNELLY would have had a relationship with Mary Ann MASON.]
* James DONNELLY (Policeman) may have been in Wellington area prior to going to Palmerston c. 1877 ?
* James DONNELLY and (real) wife Mary Ann, obviously reconcile at some point after their 1879 Wanganui incident. [I now think she is the Mary Ann DONNELLY on e/rolls in Wellington at Webb Street with Marshall. And although not mentioned in James DONNELLY's obit, she is probably the person who placed the "In Memoriam" notice (from wife and son) in 1901. We know now she is the Mary Ann (nee FOX) who goes on marry William BROWN (Nov. 1901) - so is likely the same Mary Ann FOX who married a James DONNELLY in 1864. ]
* ... "my dear wife Mary Ann DONNELLY" ... who was left an interest in the Estate of James DONNELLY (will dated 1898), is seemingly his legal wife, Mary Ann (nee FOX) ?
------------------
Gee ... why couldn't DONNELLY's wife have been named
something other than "Mary Ann" ?? Woulda saved us an awful lot of head-scratching ...
