It seems that "some" 737 NG have not been built to the required spec.
I do believe that the NG did get a few increases that would increase airframe stress and fatigue - but will that make them unsafe?
There is a chance that the previous versions of the 737 could also operate with the NG increases, but it just removes the error of margin.
History shows that JAL 123 flew very safely for along time after an incorrect repair. In fact they did the calculations on how long the incorrect repair would last before it would fail.
It safely flew something like 20% longer than the calculations determined failure would happen.
So yes 737 NG's have been safe as was JAL 123 after the repair that was not to spec.
Will this none spec construction ever lead to a structural failure earlier - hard to say without doing the actual calculations, but for that exact out of spec conditions would be needed.
As for the break ups of the fuse in those crashes - seen a number similar in non 737 and think even in other 737 types so not sure that is related to the construction specs.