I have almost an identical situation. Two glow plugs (cylinders #1 and #2)came out easily on my '99 OM606.962 turbodiesel engine. Three others were sheared off and the sixth was almost all of the way out.
At 260k miles I reasoned that it made sense to remove the head and get the glow plugs removed by a shop with the capability to do that as well as a valve job (guides, seals, grind valves/seats). The shop ended up doing electrical discharge machining to remove all glow plug remains and leave my original threads intact in the aluminum head. Then they "hot tanked" the head and all carbon was removed. It looks like a new head inside and out.
Everything goes back together this weekend I hope! As part of this "opportunity", I also changed all of the hard plastic fuel lines (6), timing chain and guide + tensioner and had my fuel injectors rebuilt with new nozzles and calibrated. Also detailed the engine bay and replaced a power steering hose that was leaking.
The exhaust manifold was sandblasted and I applied Calyx manifold dressing. The 12 year old manifold now looks better than it did on the day I took delivery of the car when it was new.
If anyone wants pictures of anything please let me know before I put this all back together.
Ken

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2014 E250 Bluetec 4-Matic
2014 GL350 Bluetec 4-Matic
1996 Chevy Tahoe 4 x 4
2009 ML350 (sold)
2005 SLK350 (sold)
1999 E300 Turbodiesel (sold)
1998 ML320 (sold)
1991 300D 2.5 Turbodiesel (sold)