No, they take big risks to try and get a big accomplishment under their belts.So...they turn evil?
Absurd.
The fact that the writers already outlined what kind of person Nog was.So who is to say Nog didn't refuse a promotion?
There's a shit ton of assumptions here, especially around worth and value without any proof.
Because we've never seen a non-commissioned officer promoted.Why would it be different? You move up in rank and responsibility.
Thus we can't say with any degree of certainty how it works for them.