Honestly if they would have redeemed the trio just like that, it would have dragged down the finale for me. And like you said, everyone has their limits. Including the princess of friendship herself. It pretty much took all of Equestria to stop them. You think it would of been all peaches and cream if Twilight would have given them the typical friendship talk? No! Even after they've been depowered Chrysalis was still talking shit saying "You think friendship can save you?" At that point, just throw the book at them and be done with it.
And now moving on from the trio to Discord, should he have received punishment? I mean, he was the one who brought them together plus King Sombra right? I don't think he should have gotten off completely scott free in all this. Even though he was remorseful about it later on. I talked about it with my friend last night, i think a fitting punishment would have been him getting stripped of his Equestrian Pink Heart Of Courage medal he received at the beginning of S7. That's if they didn't want to throw him in a jail cell.