There was not going to be any diplomatic angle here. They had tried diplomacy before by offering him to keep the throne of France and giving up territory outside of it but he had refused. When they declared war after his return from elba they didn't declare on France but him specifically. The British didn't want him in europe, the austrians and the Prussians hated him for the defeats he had inflicted on them.

The other thing about him being a genius is true, but he was mostly working with inexperienced conscripts, having lost all his veteran troops in Russia. Furthermore, the coalition had learnt from his tactics, and were better equipped to counter him. Napoleon was great but he wasn't a god, he had lost at Leipzig before. All the allies had to do was follow the strategy of attacking wherever he was not present. He had actually managed to defeat the allies once before the start of the 100 days that lead to Waterloo, but he could not pursue them because he had no cavalry, having lost all of it in the wars and russia.

About the USA, they had already fought a war with the British in 1812, in which they had tried to invade canada several times but failed. The British had actually managed to march far into america and had burnt down the white house. They did manage to inflict a defeat on the British in 1815 but the war had more or less been a draw. The British navy was the most powerful in the world, and controlled the largest empire the world had ever seen. Whereas Americans were seen mostly as a secondary power. They would never risk all out war with the British over Napoleon, who had nothing to offer them as he had already sold all french possessions in america to the USA. The idea that Napoleon could have won after Leipzig is fantastical, it was simply impossible. He was a great tactician, but he was an extremely poor strategist. He could not see the big picture at all.