If you're trying to swap the left and right mouse button it can be done with some XML magic.

Modify Divinity Original Sin\Data\PlayerProfiles\Default\inputconfig.xml:
Code
<IE_MouseButtonLeft pikey="MOUSEBUTTON1" device="DIMouse" filter="Tools;Debug;App;KEYLISTENER;Camera;Widget;UI" state="pressed;released"/>		
<IE_MouseButtonRight pikey="MOUSEBUTTON2" device="DIMouse" filter="Tools;Debug;App;KEYLISTENER;Camera;Widget;UI" state="pressed;released"/>


Swap the MOUSEBUTTON1 and MOUSEBUTTON2 values.

In Divinity Original Sin\Data\PlayerProfiles\EoC\inputconfig.xml:
Code
<IE_Action1 pikey="MOUSEBUTTON1" device="DIMouse" filter="Game"	state="pressed;released"/>		
<IE_Action2 pikey="MOUSEBUTTON2" device="DIMouse" filter="Game" state="pressed;released"/>


Swap MOUSEBUTTON1 and MOUSEBUTTON2 here too.