Not quite sure what you mean by "configurable mouse", I never had any reason to try to change the way the mouse functioned because it seemed fine to me.

The only functions(besides combat) that I can recall needing more than 1 button to be pressed were when you wanted to walk slowly(forward\up+whatever button you had walk assigned to), or to pick up items(forward\up+ left mouse).

I don't remember what the default key mappings are because I always remap everything to the number pad for 1st\3rd person games. I move WASD to 8456 and all the various actions to the surrounding keys, then I just slide my keyboard a little to the left.

*Edit* Mmmm, is it possible that they simply switched "Ctrl" with the left mouse button for the US version?