Ouch, you really need a 1.5 ton unit and unfortunately there is nothing you can do to get rid of the extra 1/2 ton capacity you got. Slowing down the furnace fan will just cause it to freeze up. A whole house dehumidifier is quite expensive, uses a lot of electricity and is basically a bar fridge with a fan attached to it. It needs to work off your ductwork as a bypass unit and I doubt you can get one small enough for your house. Good for Florida etc but they are pricey specialty units. Best bet is to get the proper sized AC and maybe you can sell your existing one with Craigs list etc.