That same bolt took me two hours to figure out how to take it off!! No normal socket would ever fit in there.
I was lucky that I have a X socket (?) (those with 4 sockets on a X) which has a very thin wall from a Tamiya RC model car laying around in my toolbox - PERFECT fit. That bolt is the same size as the bolt that locks the wheels on RC cars. You can get that from any RC stores.
To remove the broken screw/stud, you need the Craftsman 4 pc. Drill-Out Power Extractor or something like that. I've used them for a few times, pretty easy.
Hope that helps