Don't see any pics.
I can't see the "C" terminal in the #3 picture but assuming it's there, I'd connect the available spare blue wire to it. Then disconnect the red wire from the "C" terminal at the stat and make it a spare. Then you can use the blue wire for the new stat and be in keeping with the standard color scheme of the industry.
I'd think that it would be possible to control a dehumidifier with any smart thermostat that is listed to control a humidifier. It won't be able to do it without some help from you though. You would need to supply a control relay to be able to turn the line voltage dehumidifier on and off with the control signal from the thermostat.Thanks will double check the hookups and probably use the blue wire for the C. Anyone have any experience with the Emerson Sensi thermostat? Can you hook up the dehumidistat control to it as well? Was hoping to find a smart thermostat that i can do this to to eliminate that extra control.