Very interesting indeed. I can't say with any certainty that the older wing was the reason behind a more conservative approach to ops in Vietnam. What I do know is that the Aussie crews were fitted with optical bomb sights which their USAF counterparts in the B-57 did not have, hence they were able to visually bomb from straight flight and low level rather than dive-bomb. I have been told that in some of these sorties they received shrapnel damage from their own bomb runs, so pretty low. Nevertheless, yes, they would most likely avoided low-level, high-G strike action like the B.15/16 were intended to fulfill. I will happily be corrected if this was also part of the RAAF Canberra role.
Whatever the case, the Canberra wing fatigue was certainly an issue for the Rhodesian Air Force, and stress was a considerably limiting factor for them. Nevertheless, when you consider they were operating essentially unmodified B.2s under sanctions and a handful of original spares, their technicians worked miracles to keep as many flying for as long and under combat conditions for so long. While not the only non-RAF operators to modify and adapt their Cans for local demands, I think the RhAF were pretty amazing in what they achieved in making the Canberra a viable COIN bomber.
I am assuming that the Argentine and Ethiopian B.62 and 52 had strengthened wings for their underwing payloads, despite still being based on the B.2 engine series.