Will AI cheat economically as in EU3?

[Chaingun]

Off the top of my head, some/all of the AI logic in Civilization IV, Warhammer 40k: Dawn of War and Space Empires V was exposed to modders, and the AIs of all three have been modded and improved as a result. I'm sure I could find other examples if I went digging.

Indeed.

Also open sourcing (won't happen) or DLL access (to make an API for most languages) aren't necessarily the best/only options. An interface to a scripting language like Python or Lua can be easier to implement (and leave more control to game, it's not possible to control what goes on inside DLL once you call a function in it).

The bottom line is Paradox has limited amount of time to spend on AI until release, and after a bit of patching they move on to another project. Modders who really love the game remain behind and improve what they can, which in the end often will surpass the original content. The Dark Crusade (W40k) AI mod is a fine example of this; it plays nearly as well as a skilled human now without much cheating. (Whereas original AI is braindead.)