Anyway after I come and cried to the forum, I thought like "What the hell, I will give it a shot...", and continued my game. And it all turned amazing!

Edo turned to Imperial side as I said, but all its lands were at the heart of the Shogunate clans. I blitz its 3 provinces, and took one more later. And northern clans took the other one.Since it has one province left, I made it my vassal.

It all were looking good, but because of my tons of bad experience, I was like "Okay, now endless stacks of Tsu and the others will arrive and the game will turn impossible to win again". But guess what? Obama and some other clan took Tsu out. And all Shogunate clans smashed the Imperial side, both on land or at the sea, without my help. My allies are so strong, I have walk from Mino to Aki to attack the nearest enemy!

Because of the over-imbalanced AI, all enemies still ignores the other clans and sends their stacks with fleets to attack me. For example Tosa attacks Edo, and the 10 provinces between me and him are all Shogunate. But its ok. I passed the critical point.

I think I will finally win my first FotS game ever.