Very true on the second point, on the first I'm not sure - the t-stat stays closed till its warmed up regardless of whether you have the heater on. Only diff is flow though the heater core vs/rad. Having the heater on shouldn't alter warm up time much if any.
If you have an automagic car you could put a toggle switch in the dash to disable the AC compressor clutch easily enough
One area where the heater will improve mpg is if you are in the city and the motor is hot enough that the cooling fan or fan-clutch is doing its thing