S2 can be a bit unsettling coming form previous titles, I can only recommend this, itīs the best I found and most of it is still up to date:
http://forums.totalwar.org/vb/showth...-War-Shogun-II
As far as I remember:
-The CAI is not schizophrenic. The political situation in Japan is very complicated and it is very easy to do mistake and get everybody to hate you. you can always have a look at the "factors" influencing your relationship with a given clan. Among the most importants: religion, size of your empire, marriage and relationships with other clans (the enemy of my enemy blablabla)
Compared to M2, you will have to get really active on diplomacy. Try to get your daughter married, if possible, and donīt hesitate to give small gifts. Be aware also that at a point something called "realm divided" will strike Japan and put diplomacy under huge pressure.
-I am not aware of the multiple attack order issue, I cannot help you on that.
-Battles feel strange. Definitely. The pace and rules of battle are much different than the one form M2, and force you to think in a different way when approaching battles. If you do want something closer to M2, many mods bring it back close to it. I think the most praised is Darthmod.
-You can create your own formation with the "g" key. I always mix up between RomeII and Shogun 2. I think for shogun 2 "ctrl+g" keeps the formation as it was when you created the group.
-The units are more spread out than M2, especially in RoS. No way to go around this AFAIK.
-Incentive to use better units? Well, your power/space ratio increases, opening different tactics on the battlefield. However, compared to other iterations. There is no "better" unit strictly speaking in S2. The ashigarus are more cost efficient and easier to replace, but more prone to moral shock and limited powerwise. Samurais units on the other hands come at a hefty price, but are much more resilient and can open breaches in the enemy lines, disturbing the plan of battle. Two very different ways of playing battles.
Have fun