I had a similar problem on a 200 Merc EFI. Would idle in neutral but died in gear. Running it on the earmuffs in the driveway you could pull spark plug wires off one at a time and see that cylinders one and two were dead, three and four were barely running and basically five and six were the only ones carrying the load. Note that it ran fine on plane so this was apparently only the situation at idle.
It turned out to be a broken reed valve (fiber, not stainless) in cylinder number two which affected the operation of the adjacent cylinders through the plenum.
Not saying that that’s your issue but just food for thought.
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