I know you said you didn't want to use scripting... But I have discovered how to create units for the AI at game start, and I have had a good play through so far where the Major Clans have not fallen to minor aggression. (2-3 units should be enough for each major faction that is not the player - this is how I have it set up)

I am using shroud removal to make sure it is successful, and as always, am testing it as Chosokabe.