I would start by capping the fresh air vent off temporarily and seeing how things go. It should have some type of filter and enter the air stream before the filter.
If its too close to the condenser and pool heater, maybe they are pushing a positive pressure of warm, humid air into your home, even when the a/c running is not drawing it in.
And you mention more than 30 visits from just one contractor, you must have more info to share than what you have so far. Has anyone looked inside the air handler or supply plenum for mold? That would probably be the first and most likely place for it. If its always damp and all your air is being cycled through those conditions.