Well I went ahead and uploaded my changes. I tried various ways to get the infantry to fight better in formation, but simply ordering the infantry to hold fire and charge when the enemy is near (any enemy within 8 meters, enemy infantry or archer formation center within 15 meters) seems to be giving the best effect. They move across the field in formation, throwing weapons and such while approaching or facing off against each other, then swap to melee weapons and charge in.
I did the hold fire part because otherwise the charge AI was making half of them rush in and the other half stand back holding throwing weapons trying to get an angle to throw them between the other half to hit the enemies. And the charging infantry kept getting in their way, so the total effect was half the infantry rushed to battle and the other half stood back like "So, are we charging now? Is that what we're doing? I'm confused."
Let me know what you guys think. I ran various test battles and it seemed to work ok, but I'm sure it still has its issues since all I'm really doing is disabling all that fancy infantry formation AI that Motomataru spent so much time on every time the infantry start fighting. Well, not ALL of it - I situated it so that they still fall back to reform if they get really spread out (mostly when reinforcements arrive).