Very well written Ison. Many kudos to your great research into this topic.
I want to bring up an oft observed but little discussed point however. That is, the game AI does have limited capacities to planning more than one turn ahead. We see this most often in how they place units before they go to war. Sometimes, sending them over fvst distances (several turns) before declaring war.
I'm inclined to believe there is some subsystem at work that allows for these type of planning. Granted a player can reload and save and sometimes prevent and invasion by moving things around, which again would indicate a turn by turn planning as discussed. But that would only suggest the old 'battle plan' was scuttled because of a new development. As another poster noted, the AI's strength and weakness is its ability to abandon plans as things change.
Because a lot of things going on aren't directly observable, like what technology to research next and or why the AI likes to sometimes send fishing parties into your territory (this is different from those units crossing our territory to go to war), it is hard to say definitively how deep this planning extends into the AI decision tree.
I strongly agree that the bulk of AI planning is made in one turn for the next turn with the whole cycle starting again and so on. I still believe that it has some capacity, likely scripted and very limited, in planning several turns ahead. Although the end result could be that the plan is scuttled.