Just in case you all thought that I had given up on this one.
I have come upon an interesting piece of information that adds a tiny bit more substance to the arguments we have been pursuing.
I previously noted (30Sep09) that ""In January 1876 Benjamin Watson, carrier of Adelaide, bought 10.25 acres (4.1 hectares) of section 87 (Speirsville) from the estate of the late Samuel White, butcher of Plympton."
At
http://archivemaps.com/mapco/adelstreets/streets18.htm there is a 1929-era chart that shows section 87 of Speirsville. Interestingly, there is a locality of Black Forest, (the 1853 birth registration locality of Mary Ann White) that borders the property sold to Watson. It's been quite a while since I lived in Adelaide, but I believe that this is a bit further west of the modern day location of the suburb of Blackforest.
So ... while I still can't pin down the elusive Joseph Frederick White, the Samuel White/Margaret Walsh marriage that produced Mary Ann White of Black Forest is indeed the same pairing that lived at "Bay Rd Plympton", the birthplace of Joseph Frederick White.
As Pink Floyd sang ... all in all we're all just bricks in the wall.
So where does that leave me? Getting closer but still not quite there.