Good summary, Bob.
XXX, the Golf parts catalog is available at the same site you visited:
http://www.ultraleicht.de/tecnam/PDF/sparesP96_GOLF.pdf
I'm not an aero or structural engineer but it looks as though the failure line is outboard of the doubler you point to. Just counting back the rivet lines from the tip, and noting the amount of aileron showing.
Maybe not much ultimately relevant in the structural analysis of what's clearly a pretty reasonable airframe, given all the other more pertinent points of weather and possible loss of control scenarios raised. Until you know it was definitely failure under load as opposed to e.g. collision with a wire or structure, it's all pretty speculative.