This is quite an exciting scenario, ships vs wading troops.
The only quibble I have with this scenario is the Victory Conditions, in that the all or nothing Minamoto victory doesn't allow much comparability.
If I was setting the VC, I'd give one Temporary Victory Banner for each Minamoto unit that starts the turn on Kojima Island, plus one banner per Taira unit eliminated (as usual).
“I dreamt of worldly success once.”
― Miyamoto Musashi