Number 5 actually helped a little! Thanks.
@4 I didn't find a maxtexturesize option in rendering.ini. Maybe I should create it.
It's actually in "User.ini" as well, under [Rendering] section. Two more values to check there, they should be set like this:
"AtlasTextureDownscale=0"
"DetailTextureDownscale=0"
One more thing you could try is go to nvidia control panel, create a profile for your Witcher 2 game and experiment with those texture filtering options.
Other than that I'm out of ideas... full reinstall perhaps, although I doubt that would help.