Having the AC button lit on the screen doesn't mean the Prius uses the AC. So you shouldn't have to worry about it.

As for heat etc. I find that the prius does a good job if you keep it at automatic. I only uses the front/rear defroster during the first few minutes.

I have found that turning the heater off when you are coating to a red light or standing still can improve your FE. I've started 'pulsing' hear into the cabin using the A/C Auto button on the wheel and it has helped my FE queit a bit. I am mainly driving on 35 miles per hour roads and slower so for me it makes a significant difference if I can coast with the ICE off. If I go faster (like on the highway) I just keep the A/C to auto.