The 6 months axiom is based on the premise that the clock gets reset on GC approval as essentially GC is based on future employment. Now there is no officially defined time period how long you need to stick around but 6 months plus is good enough since that is what you are expected post I485 filing. What can help is the total time spent with the employer and time post 485 filing as you mention.
You are correct in saying that changing right away after GC may cause a red flag / audit etc. You are also correct isaying that employer compalining is like shooting themselves in the foot.
Regarding filing I9 make your employer do that because that's the offiial record that you are working on GC. You are correct that grand fathering may happen once you have received citizenship. Your employer should ask you for a copy of your GC to file.
On another note I have seen many people leaving their employers within days and months of getting GC but havent read of any examples of any GC revocation or problems during citizenship.