Historically, Ranmaru was loyal to Nobunaga to the end. Mitsuhide was the one that betrayed him and forced him to commit seppuku. Yoshiaki the bratty little shogun here raised an army against Nobunaga. Historically he most likely did it because he felt threatened by Nobu’s popularity among the people. Here he did it for that AND love for Ranmaru.
I hope this fictional story ends so that Ranmaru and Nobunaga “died” in the famous last battle fire. As in Mitsuhide spared them out of love for Nobu and let them run away. Not exactly the warrior ending for Nobu but I’m thinking maybe he got so wounded and weak that Ranmaru just took him and ran