Comment 30 for bug 1590590

Regarding my post #24, I think a better workaround, instead of changing .Xmodmap,
 is to add the following line to the file:

~/.xinputrc

xinput set-button-map "ImPS/2 Generic Wheel Mouse" 1 2 3 5 4 6 7

in this way only the two-finger scrolling direction of the unrecognized touchpad will be inverted, while the wheel scrolling direction of an external mouse will not be modified.

In addition, due to some bug in unity/gnome not all the applications interpret correctly .xinputrc/.Xmodmap commands. So for example chrome/firefox will be ok, but nautilus windows will still use the "natural" scrolling, regardless of .xinputrc/.Xmodmap content.
I did not find a solution to solve these inconsistencies.

The only real solution will be of course a proper support of our touchpad at kernel level