check out
this tool
this is the kind of tool i've been describing; it turns a socket at a 90° angle you don't need to have any back and forth motion; it also has little bit of reach, i think it might just exactly make the corner.
i was looking for a geared version but this one is strangely gearless.
I think if you got the 1/4" drive version it might give you a little more room to work.
-awr