Plugging the entire mess off would probably be the easiest plan.

Removing all of the remnants, and reinstalling a new glow plug would be the best remedy (if it can be done).

Are the threads for the GP still intact? If so, a new glowplug could be coated in release agent, and oushed into the hole with the breached area packed full of JB Weld or equivalent (probably a high end industrial epoxy made by the likes of loctite).

With a little luck, you'd have the same profile as the original hole, with just a small part of the bore being epoxy. You may be able to even have a welder tig that coolant passage closed before this epoxy reconstruction.

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2017 Chevy Colorado Diesel (nanny state emissions)
2005 Volvo S40 T5 AWD, 77k
1987 Mercedes-Benz 300D turbodiesel, 4 sp auto, 156k - 28.7 mpg
1996 Tracker 4x4, 2 door, 16v, 3 sp auto. 113k - 28.6 mpg

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