Why not remove the other two bolts and see what you are left with. I would think since the head has been flattened the housing should come off. I don't recall if the housing itself is threaded. Does the replacement bolts have threads all the way up the shaft? They are aluminum bolts as I recall. They can easily be drilled out.
The flange is left on that E10. My friend had ground out the head but not the flange part. For now, he has decided to drill it out.