Having posted the referenced lengthy account of my GPU issues above I need to retreat the line of argument that it falls into the same issue category as my MBP and is not the by others faulted Early 2011 model.
My MacBook with the issues described is a:
Retina MacBook Pro Mid 2012 a.k.a.
MacBookPro10,1 Sales# MC975LL/A, Model# A1398, Production week 27-2012
with a discrete NVIDIA GT650M chip.
The issue of a lack of transparancy on HW issues I can nevertheless underline from having an issue-ridden MBP from another prior episode. The (first) Intel MBP 2006 I had in use produced a high pitch whining noise which similarly resulted in 2 mainboard swaps. The time it took until it became an acknowledged issue with service being aware of it was lengthy, and the customer experience then mimicked the one described before...