I challenge you!

I'm suggesting that you should try... using StartCombat, the player should have control over this hero from the reserve.

Well, it won't work in single-player mode, and it will even change the checksum if you transfer the side during the battle. If you add the first player to the reserve and have them fight the second, the hero from the reserve will be neutral. If you're not talking about these two options, please clarify.

Added after 57 minutes
I'll try another option: a Castle in a Dungeon and initiate a battle with it.