Successor game rules, draft one.

[TinCow]

I think you've misunderstood the rule. The "player whose avatar leads an army that is involved in a battle" IS the player that the game chooses as the leader. There is no IC/OOC split in that rule, the person who shows up as the leader when the battle appears in-game is always the person that fights the battle. That's how it worked in LotR, and KotF 1(c) is a direct copy of the same rule in LotR, marked there as 1.3.

On reread, I see I misread your post. However, I dislike using the console for all of this. There is a very simple solution to this problem: if you want to fight the battle yourself, don't merge your army with an avatar that has a higher command rating. If you absolutely MUST have that other avatar coming along, just park them outside of the stack before the battle so that they join in as reinforcements.