Set up a custom keyboard layout, or use
Autohotkey to remap the right Alt/Ctrl keys to function as the left ones, or swap mouse buttons, etc.
You can also swap mouse buttons by hex editing the Engine.pak file, in the '..\Divinity - Original Sin\Data' folder. If you want to make a backup of this file, do so in another folder; having extra pak files (even renamed) in this folder can cause the game to crash on startup.
In a hex editor, such as
XVI, open the Engine.pak file, search for the text string MOUSEBUTTON1 and change the 4 instances to MOUSEBUTTONa (either manually, or use the search and replace function). Next, replace MOUSEBUTTON2 with MOUSEBUTTON1, and then replace MOUSEBUTTONa with MOUSEBUTTON2.
