1. Limit the fresh air ventilation to times of people occupancy. The quantity of fresh air should be setup to avoid over ventilation. ASHRAE's 7 cfm per person + .1 cu.ft. per 100 sqft. of space the minimum amount. This amounts to 50-75 cfm of fresh air when people are in the home. During your extreme humidity, this is 3-4 lbs. of moisture per hour removal while maintaining 75^F, 50%RH. A properly setup a/c will remove the moisture during the heat of the day. During the evening hours when the a/c load is low, a good whole house dehumidifier is needed to remove the 2-4 lbs. of moisture per hour to maintain <50%RH.
2. During the days of setting the a/c up to +80^F(no people), no fresh air ventilation is used. There is some infiltration, but because of no cooling, there much less dehumidification needed to maintain <50%RH. Roughly 2 lb./hour of dehumidification or half of moisture removal. When homes are cooled below the outside dew point, more moisture condenses on the exterior surfaces. The condensed moisture moves through the wall to the inside of the home increasing the moisture load.
3. Cooling the home down 10^F prior to the poeple coming provide a long run to efficiently remove a lot of moisture. Also critical to operate the a/c with a very cold coil. Freeze-up and duct sweating determine the minimal coil temperature.
Using the above points will reduce operating cost maintaining the summer temperature in the home. The cost of single speed a/c and a whole house ventilating dehumidifier is competitive to the full featured 2 speed a/c. Inculding the dehumidifier provides humidity control without any a/c operation. This works well. Regards TB