If his identity was known then there are a lot of ways it could've gone. Like Minato able to reason with him and convince him to return back or the Kages not taking him as seriously as they did thinking him to be Madara. The manga probably would've gone a different direction too.
Nothing, nobody would be able to do anything about it in the 5 kage summit. Minato would try to do some lecturing during the village attack, and it would just lead to more hesitations but the outcome won't change.
Nah, you saw my boi Kakashi during the war. He was ready to kill him, im sure by the time he's declaring war or interrupting the 5 kage summit he would have solidified his will.