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you told us several times you have never had an 8mm nipple seize so I guess there is only one way forward.Good luck.
Left hand drill bit?Or new caliper from MGcycle go for about $150.
Next time you have a stuck nipple in a brake caliper put the caliper in a vice, place a box end wrench of the correct size over the nipple, take a brass hammer and slowly and without too much force tap down on the top of the nipple, over and over while applying slight pressure to the wrench, eventually the nipple will come out but the nipple will be junk by the time you are done.
I love these little bleeder repair kits , come in various sizes and have a brass NPT seat so no worries about destroying the seat when drilling, You could probably find a core bit that would drill right around that hardened mess in the center.
i dont know what size easy-out is broken but saw a siilar problem on another site and the chap resolved it by drilling with one of these https://www.ebay.co.uk/itm/134101140059?hash=item1f390d025b:g:C-UAAOSwfwphHjiZ lubricated with water and kept taking small bites at it.may be worth a try?
At this stage of the game I'd go to a reputable workshop and see if they do spark erosion.