Selecting Dark Vision on the hotbar everytime I want to enable/disable it is pretty annoying. Is it possible to bind a key to toggle this power, e.g. by editing one of the ini file?
You can map any skills from the Skill Wheel to the number keys, just open the Wheel (using Middle Mouse Button), select a skill and press the number key you want it have assigned to.
So in short:
Since Dishonored is an Unreal 3 engine based game, you can try setting up an alias which will chain two commands: switch to a particular skill (or at least press a specific number key), attack.
See the ue3 docs: ("key bind chaining" & "key bind aliasing")
You can probably derive the commands for left-hand/right-hand attack and skill selection from the INI file(s). At the very least you'd be able to automate the 1-9 selection and attack, and then fill the slots manually.