Can a mod access the ENTIRE knowable game state?
Can it force the AI to make exact moves based on arbitrary custom logic (i.e. intelligence)?
what you mean by "moves" ?
units moves ? yes, with limits
diplomatic moves ? same answer
Does this logic have to be written in a specific language?
Will the expansion affect these things?
The expension, no. The release of the DLL source code, yes.
AFAIK the source code is not planned to be release at the same time as the expansion, but later.
What will it change ? replace "no" by "yes", "yes with limits" by simply "yes" and add c++ to the available langage (depending of what you want to do, combination of both may be required).