Hm. I have everything that you suggested checked and still no horizontal scroll ability.  I even tried manually adding Sins to the list of games in the "advanced game settings", but that seemed mess with my mouse resolution in the game for some reason, so I removed it.  Everything else works fine - I can double click, I can hold down the wheel to move the screen, I can assign a function to the extra button on the wheel, etc.

You sure you can horizontal scroll in the game? Did you manually assign that to a button?

Hmmm!!!  Well I'm out of ideas.  Thanks for looking into that!

In assigning key functions, strafe left-right are assigned to left/right on keyboard but when I try to 'add' a button and then hit left or right on the mouse wheel, nothing happens.  It's like its not being detected.  *sigh*