Stability testing is really up to the tester to determine if the computer is stable, some feel 12 hours of prime is needed w/2 hours OCCT etc.
Generally I just run 1 hour of OCCT and have yet to have any problems (But thats just me) Followed by alot of everyday stuff and alot of benchmarks and if no issue's arrive...I call it good.
As far as you northbridge temp and voltage....I cannot really say, but if you can lower that temp the better off you are (Not saying your gonna hurt anything...not saying you won't).
there's really not a whole lot of answers for that kind of question, But if the board doesn't shut down it can't be that bad.
One one of my 940 builds I have the NB @1.4-1.42v and it doesn't seem to be hurting anything.
1.5 Vcore is just fine for air