Hi Michael, for the sake of the argument try a new spark plug, just because they show spark at atmospheric pressure doesn;t necessarily imply they will spark under compression.
Secondly, how flooded is it? If you pull the plug out and pull the starter (turn it upside down so the plug hole is pointing downwards), do you get much liquid coming out? If so, let it dry out for a day or 2.
Beyond that everything else you've tried sounds logical and methodical.
