I don't agree with you, from your point of view, you take the worst case scenario, if a player wants to throw the game, he will find a way to do so. The only way that the player could control the army, would be in the manual battle, no in the campaign map. In the manual battle, the player choosing to complete a manual battle, is the one in control on the location and time the battle is completed, not the enemy player. It could happen, but the likelihood of it happening is extremely low.
It would be boring, this is why the option of gifting an unit would be there.
Why would you not want to gift an unit, like I said I play with my brother, if you play with randoms, it would not be the best case scenario, this is why there would be an option to allow manual battle or not in the multiplayer lobby. And if you play with friends/family, why would you only give peasants to them, you could share and play as a team in your own army. He/she could control the cavalry, while the other control the infantry, same thing with archers.
And it is sure workable Total warhammer did it, and its great.