Some comments:
1) You don't need to hold the button down, just click at a location and the char will walk to it (although sometimes you have to click multiple times to make it move)
2) Seconded.
3) The environment *is* 3D, but limiting the angle of rotation simplified the map design process (or so I've heard).
4) 5) The full list of keybindings is in the Divinity - Original Sin\Data\PlayerProfiles\Eoc\inputconfig.xml file
I think the mouse buttons are remappable by changing the <IE_Action1 pikey="MOUSEBUTTON1" binding

6) Noticed this as well.