Birthright: Chimaeron

The point isn't you had to be perfectly optimal to defend. You just needed to do anything at all to defend. Or you can opt to spend 6 actions and have a bunch of Hero-Kings running around fighting instead of an army. That is resource-efficient and action-inefficient.

Do leadership units all have "Scout" movement?
Can they be "Blessed" and then move unlimited distance from the blessing to join a battle?

The solution is to convert Scout into +2-3 movement not ignore terrain movement rate. All races can build Scout-swarms. You are focusing on the wrong thing. It is the scout ability that is the problem. It is not the Sidhelien units all having Scout.

I would note that I have not been sending waves of Irregulars. I agree that that would be more optimal than what I am doing.

1 person - no matter how optimized cannot beat 10 people. Ever. Actions are worth more than any amount of efficiency.

Any race could have gone all-scout. Archers are marginally better than scouts - yes. But it is not that huge a difference. Karamhul should not be spreading out like crazy. It is not a good racial-strategy. They should go tall - they are impossible to dislodge from a secure position.

It just means that everyone goes back to building only scout-armies. A better solution for long-term viability is to make the "scout" function grant bonus movement not ignore terrain movement cost. An all-scout army of any race can move through all terrain for 1 movement cost as it is anyway.

I was thinking you subvert the lieutenant to lead the army somewhere else. They are in command of the army - it seems a reasonable action they can take within the rules. But maybe not.

If someone leaves a large army under the command of a lieutenant (lieutenant action led war) - it is a devastating spell to use.

I think #1 is the most interesting. #3 is second best. I think #2 is boring. You make Subversion simply another route for the person with the most RP to always win at everything. We already have lots of those.