Steam has released the patch to help its users evade the notorious "You are running 10.6.8, and need 10.7" error notification, however the error notification still appears.
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I deleted the local files (in the Steam application, by right-clicking on Crusader King II in the list of Steam games), and then re-downloaded the game through Steam, and the problem went away (I can now launch in 10.6.8 with no error asking for 10.7). Admittedly, it's an inelegant solution, and your mileage may vary. Good luck. 
If you launch it via the Steam launcher is the flickering problems and slow downs still occuring?
My computer's video card is underspec for the game as it is (ATI Radeon HD 2600 Pro), so it does run slow. I don't think I'm experiencing the flickering problem, though. But I don't think that has to do with me launching it through Steam. Sorry I can't help more.
According to post #2, they have. I myself had to do that too - delete local files then reinstall, and I then got past that situation.