I appreciate that this is way off line from all the knowledgeable people trying to help with your problem, but I have been following your thread since the beginning and am reminded of a bizarre experience I had.
I picked up a boat that had been left in a Marina by the owner, it had a small Yanmar which the owner told me had been running perfectly.
I wouldn't start, the engine spun freely so I did all the relevant things that you have done, checking fuel,air filters,etc.
Then I got a mechanic, same story over again and eventually "he thought" it was low compression and the valves needed reseated. Removed the head etc, No difference, next up it was supposed to be piston wear.
By this stage two weeks had passed, the owner was furious that her boat hadn't been delivered, so with nothing to lose, we took the exhaust elbow off and it was blocked solid.
Possibly one more long shot to rule out.
Seafever of cuan
SM 425 Redline