update: i had success!!

plan A: failure. i bought a dremel grinding bit that was supposedly suitable for hard metals, and tried it. no go. it wouldn't grind through the hardened extractor. all it would do is polish the exposed stub of it.

plan B: failure. i tried tapping out the extractor from the few mm sticking out the back side. no go.

plan C: success! i used my cotter press to press the extractor out! this is illustrated below:

broken bolt with broken extractor stuck inside:


backside of eyelet showing tip of extractor poking through by a few mm:


cotter press on eyelet. press was too large, so i used a hollow sturmey archer axle nut to take up the slack. extractor popped out without effort:


next step was to use the dremel grinding bit to route out the hole originally drilled in the bolt for the extractor. i routed all the way to the eyelet threads without destroying them. i was then able to squeeze and collpase the bolt with needle nose pliers to reduce the pressure on the eyelet threads. then i was able to thread it back out! this is how the bolt looked after removal:



and finally, the eyelet after major surgery:


the eyelet is a little scuffed now, but it won't be noticeable once the fender stay clamp is bolted in place. the threads are fine, i chased them and a new M5 bolt threaded in perfectly!

thanks to all for your advice!