As it turned out it was even simpler...
We got it back to Captains Cove and the good folks there had it up on the lift in less than 20 minutes. This allowed me to remove the 10+ feet of 1 1/2" nylon tow rope that had twisted around the prop!
In retrospect the symptoms made sense. The dense black smoke was due to the huge load placed on the motor from the wrap, the power loss ditto, the vibration - obvious, the tranny issue must have been due to the shaft being unable to unload after it was placed in neutral.
All this added up to me being sure I was suffering catastrophic engine / tranny failure until I called the Yanmar guys and he told me to look for a wrap first (guess it's not the first time they heard this story).
Added bonus is that Captains Cove would not let me pay them for the lift,. Great guys.