The lever (#3) is removed by lifting up, inserting a screwdriver from underneath and pushing up to unlock it from the metal lever (this is the bit that does the work to unlock the seat back) that is inside. As it's broken already you need to remove the remaining plastic surrounding the metal lever inside.
Once you have done this just get a new part (#3) and push it on. Screen shots from the workshop manual below.
Unrelated to the control button, does anyone have a part number(s) for the Air Bag covers displayed (missing) in the Illustration of the seat back? I need to replace mine as they are sticky/deteriorating. Does the cover include the entire side Air Bag or is it just a cover? I can't find the part in my manual.