I've done the exact same job recently and had the exact same difficulties.

You can get a 12 point box wrench in there, but just barely. It took me over 1/2 hour to get that damn screw out and another 1/2 hour to get the new screw back in.

A better solution, which I have subsequently found out, is to force the piston down below the body and fender and rotate the shock outward away from the vehicle. Now the screw is out from under the shock and readily accessible.