If you are feeling brave enough you could try using an e-z-out/easy out... though i have never used one on a car, only on aircraft. We use them all the time at work on the fighters, especially as they are nearly 30 years old and bits regularly snap when trying to undo them!!
Helicoil inserts would only be needed if you have to get the sheared bolt drilled out, not needed if you can easy out it.
Not sure about access on the bolt in question, but if you can get into it, buy yourself a decent set of e-z outs from halfords/internet.
