1) What is the difference between raw and direct input?

Not much: Raw Input is more direct (ha!) and DirectInput is a little more buffered and delayed, but not so much as you would notice anything. If you have a choice, select Raw Input.

Raw Input is the message loop of the program receiving mouse movement as Windows gets it, and setting it aside until the program decides to process mouse movement.
DirectInput is a separate DirectX thread (in effect a separate program running just to process mouse input) receiving mouse movement as Windows gets it, translating it from Raw Input format to DirectInput format, and setting aside until the program decides to check DirectX for mouse movement.

2) Is mouse acceleration in CS bugged or something? It isn't uncommon in Quake, but so very rare in CS.

I don't know.