00,
With respect, I think you underestimate the differences between the GR5 (ZD serials) and the factory built GR7 to GR9 (ZG serials) and this would partly explain your views on the II+ being "markedly stronger"
The GR5 in its early incarnation, was indeed a little 'underengineered' as it were, and problems with vibration and the associated cracking of structural components did begin to occur and had to be dealt with. By the time the ZG serial GR7's were under construction there was a handle on this (possibly including your own photographic evidence!) and as a result of this the ZG serial GR7's were structurally slightly different in many areas, along with the subsequent attempts to beef it up. This becomes more evident with a quick look at the differing part numbers of structural components in the early ZD batch aircraft to the later ZG batch aircraft, with the ZG serial former GR7's sharing a much higher degree of commonality with the II+ than the ZD serial former GR5's, the early ones of which rarely sharing commonality between theirselves!!
However, regardless of these facts, the Harrier - in all forms - is unfortunately now a tired aircraft, and all are suffering from the same forms of fatigue, especially around the rear end. You would perhaps be surprised to discover the extent to which this is true, and whilst not insurmountable, the aircraft will soon be reaching the stage where it is no longer cost effective.
I could go on, however, I get the sneaking suspicion that London Eye Hospital has just had its stock of glass eyes bored to sleep!!
Flipflopman