By comparison, TOW has no AI. SOW is far and away the best AI for 19th century warfare. The AI gets no advantages or cheats. It is a true 19th century general. It reacts to the things it sees at ground level. If you play at saddle level, you are on equal terms with it.
Division sized battles will last 90-120 minutes on average. Corps and army size battles can last 2-3 hours easily. So maintaining reserves is not just a good idea, it's a necessity. The AI certainly thinks so. Just when you think you have the upper hand in a battle, the AI brings in his reserves. If you don't have any of your own, you are doomed. I am referring here to sandbox games where the AI behavior and battle are not scripted. It is the best feature of the game IMHO.
The AI does use some terrain such as fences and slopes to it's advantage much of the time. It doesn't do as well as a human mostly because it is biased to attacking rather than defending. If it sees what it feels is a better spot to engage the enemy, it will leave its position and move there. This is probably the most complained about behavior in the game. If you really want a regiment to stay in one spot and not move, you have to TC it and turn it into a zombie.
The most important thing to do before a battle begins is to know what are the capabilities of your sub-commanders. Those attributes such as command, leadership and style are not eye candy. They determine how he will fight the battle. Better generals do a better job. Aggressive ones tend to attack in nearly any situation, while defensive ones do not like to stick their necks out. Competent generals will tend to execute their orders well, incompetent ones...well good luck. This is the second most complained about feature. Many beginners think that they are god and all orders they give will be executed immediately and perfectly. That is not how human generals behave in battle and the AI doesn't either. You will need to develop a tolerance for frustration. It is one of the best features of the game.
One poor feature of the game is the wandering supply wagon. If left on its own, it will travel up and down the roads and through the regiments on the firing line. I always TC it and place it behind my lines where it won't get into trouble.
I think that answers all your questions.
Secession is futile.