Francois Vander Meulen
Question Nr. 1: Should I add another -1 modifier due to the river for Shingeki vs Yari Fusuma ?
Question Nr.2: Is it correct that I only need to consider the highest modifier of the two attacking units instead of adding them up (and getting -3) ?
You mean the modifier for the attacker's units state ? Yes, only the highest apply.
Red dice modifier: 7-5+1-2 = +1
att. Elan+Masse(+7) + def. (Elan+Masse)(-5) + att. leadership(+1)+
Note that the leadership modifiers can be applied to the red or blue dice (each player can choose for his units).