UPDATE
Since the saga in post #2, I have put another 250 hours on the
L45.
The nickel never-seize did its job; this time the plug loosened with moderate pressure on a 3/8 wrench. The recess in the plug was partially filled with dry particles of soot (large, dark patch near the plug). When I rapped the muffler some more black dust fell (the smaller patch near the middle). I put a clean rag over the hose of my shop vac before attaching the nozzle, but I clumsily sucked some flakes into the vac and further disturbed the flakes when staging the picture. The total amount of soot was certainly minimal. (What might be taken as oil stains on the L rag are floral prints on the former bedsheet. No liquid was found.)
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I suspect the spark arresting muffler addition and service instructions were an afterthought to comply with the USDA and secure the North American market. My suspicion is based on the absence of the part in the parts manual and the slightly different style of the muffler maintance instructions as if they were written to satisfy USDA bureaucrats.