O.P. again...
My thought is get the house professionally measured, get the right size downstairs unit (12 year old house/ original unit) and then maybe seal the crawl and get a dehumidifier installed in the crawlspace? Or maybe just shut the vents and turn the crawlspace fan off with the new HVAC unit and see if it helps with the moisture content.
Oh and Mom says the crawlspace is usually dry, but it was damp when this guy went under the house, again all the crawl vents are open and the fan sucks air through.
Thanks again, John