Had this on my 09. Previous owner had passenger side break. Dealer replaced. I had driver side break top and bottom. Replaced both with thicker EV rated springs. Identified by blue dots on springs. Standard ones have pink dots. EV springs will give you an extra 1/2 to 1 inch ride height.
I bought a ‘Sealey AK6926 Go-Through Low Profile Walldrive Metric Socket Set’ to help to remove the top nut, however, that didn’t help because the Allen socket strips.
The correct tool seems to be ‘Genuine Smart 450/451/452 Suspension Shocker Workshop Removal Tool W451589006300’ off eBay.
I plan to have a go tomorrow to replace my strut top bearings. I usually remove the front but shall give your method a try.
The engineering of these SMART Cars is awful. I hate them.