And if Lee had been in command at 1st Manassas, the war would have been over as well.
But your point is 100% correct. Lee would not stoop to the tactics that Sherman & Grant used, and that was his fatal error. Nevertheless, if Lee had the resources, men, etc. that Grant & Sherman had at their disposal, the outcome would have been different. What made Lee great, was what he did, with what he had.
No it wouldn't have. Second Bull Run, Chancellorsville, Fredericksburg are all cases where Lee decisively beat his opponent and did not pursue his defeated foe. There is no reason to believe that he would have done it if he was at First Bull Run, either. He would have been faced with the same problem that Johnston and Beauregard faced - the confederate army in victory was as disorganized as the federal army was in defeat.