They werent able to do it when Edelgard was alone, nothing indicates that they would be able to do that with all the 3 powers together.
I... don't think you get what I'm saying.
Let's say all three leaders put their heads together and decide to give peaceful change a chance - a real chance. They assume control of their countries, Edelgard gets rid of Those who Slither... peacefully... somehow... without anybody getting nuked, then they turn to Rhea and say "yo we know you're a dragon we'd like to control our own continent and possibly get rid of the crested nobility system you built please leave. She says... no. She doesn't go axe crazy and order the Knights to start burning cities (though honestly there's a solid chance that happens anyway), she just refuses, and excommunicates all of them. That's in her power! She then says that their reforms are heresy and anybody caught subscribing to them will be labeled a heretic, and we know what happens to heretics. So now you've got a church headed by a dragon who won't leave who executes anybody they can actually catch trying to reform anything. Not the leaders, not even the higher-ups, but like, random imperial, alliance, and kingdom officials and nobility, the little pieces of the larger nation, the parts they can get to.
Is this a situation that gets resolved peacefully? Or do the leaders need to put their foot down and actually march forth?
The massage is pretty obvious. Shit needed to be stirred for change to happen. Now, wether or not the one stirring shit up reaps the benefits, that's a story for another day. We have a few historical examples of revolutions where the final "winners" are far from those who started it in the first place. If anything I don't think we can find historical examples of radical changes occurring without violence.
Not prior to liberal democracy, and even then, there's still usually
some violence.