This sounds like the worst-case scenario for how the Nohr path could have played out. I was already concerned the story was headed this direction when, during that E3 demo of the nohr side, Garon told Kamui to suppress the ice tribe rebellion and Kamui basically went "k. brb." Not once questioning the morality of it. Just the difficulty of the task.
Ugh...I had hoped the Nohr side would be about gathering allies(like say, those ice tribes they mentioned)and plotting to overthrow Garon while trying to deal with the invasion. You could even have had the tables flipped around on Nohr, so that Hoshido would be the invading party since Nohr would be in disarray if a rebellion broke out during their own invasion. Anything but Kamui blindly obeying Garon and then doing a heel turn at the very end.