Well God can certainly have angels appear in many forms, if He so chooses. And yes, correcting/preventing all of our mistakes is certainly not job one for them.
Correct. If that were so then this human stage in our existence wouldn't be necessary. There would be no Hitlers, no famines, no crimes, etc.
Lucifer and his auxiliary wouldn't have been permitted to exist if not for a reason.
I think of our human stage as a process of character building to prepare us for the next step.