>>23346171 (OP)>What happened to his le nihilism?It's still there, it was just part of his history so he can understand when other people are just as empty as him too.
>What happened to being empty?He mostly was, but found refuge in Char's ideal of creating a world where Newtypes can be free. Free from being used as weapons, free to pursue their own dreams, to fight for their own sake.
>Why does he all of a sudden give a shit if Char wipes out Earth?Partially because to wipe out the earth is an irredeemable sin that will blacken one's soul permanently and he doesn't want his friend to fall down that well. He can tell that Char is cut of the same cloth as him, which was how and why they became friends in the first place. So he can tell that Char is someone that doesn't have anything to lose and knows only of fighting and revenge, and so he should not be in charge of Zeon, or he'll eventually fall into the same pit of despair Challia did, the same gravity well that drew the Zabi family in to become psychopathic murderers. Even Kycilia speculated that she could've had a pure child like Casval and lived a happier life.
Him attacking his friend was a warning: "I'm keeping you from going down this road, by force." Char got the message and heeded the warning.
>And when the hell did he carry out a plot to install his sister as the leader of Zeon?Obviously behind the scenes. This was a plan long in the making, as killing the Supreme Commander and Major General of Zeon would leave a power gap that would either be filled by the Federation or some other sockpuppet. So he likely worked with Ral and Sayla to quickly install her as the new Supreme Commander of Zeon once the feuding Zabis were out of the way, to avoid that power gap.