This is not IDI specific, but I thought it might help somebody out sometime:
I had to remove the VNT15 from my hydraulic 1.6 in order to do a head gasket job. I know I had used lots of anti-seize on those fasteners when I put it together only about 12k miles ago, but still, one of the 8mm bolts sheared right off when I put a wrench on it. On examination, it looks like the bolt had been cracked for some time, as you can see in this pic.

There was a good bit of the bolt exposed, and I could have just tried a vice grip on it, or maybe even two ground down nuts locked against each other, but I have a favorite technique in these cases. I found a larger hex nut that would drop over the bolt, in this case a 3/8". Then I sawed off the bolt so that it would extend about half way up into the nut, as you see here.
The I got out the MIG welder. I set the AMPs pretty high, as heat is the whole point here. I carefully filled the cavity in the nut with weld bead, then let it cool.

I put a socket on it then and turned it out, it came right out with ease.

There you have it -- no promises, but that trick has never failed for me.
Tom