I cannot find anything to further your knowledge of Alexander but I wonder if this is another marriage for Margaret Hadden
Empire newspaper 1 Aug 1874 Report of bigamous marriage by James Mason to Margaret Hadden at Mariners Church in Sydney.
I cannot find a connection between Alexander and either of the witnesses.
He marries as a bachelor, so brings no children to this marriage, and this is an unhappy marriage, then Margaret's family would be unlikely to maintain contact with him after her death. Difficult to find anyone to connect him to.
Hard to know how you can find anything more on him but there might be something somewhere.