Unless your heat pump can be natively seen by smartthings its going to be abit harder to initially be set up the way you want it.
Variable definitions as is what? Im unable to understand what you mean by this?
Im setting temp with either virtual switch or directly in webcore by ifttt maker request.
You can either do virtual switch that sets the fan speed and diagonals of the fins or directly by IFTTT or your other options is to get a wifi / ir blaster that can be connected to smartthings or ifttt or similar.
I made a master piston which registers all my heat/ac pump settings and a second piston which has the rules and a third piston setting modes depending on time and presence.
If none of the above is an option for you, you could for example set a smartplug and make a rule for it to only work when within outside temp range.
Regarding the “low fan and diagonals” obviously we have the dB issue but other than that, the fan itself doesnt use much and its more energy efficient to reach setpoint as fast as possible than to slowly work your way up.
Auto is most likely the fan that auto regulates the speed, its utterly crap just as the auto mode is for heat/cold.
Dont know if any of this is to any help, if you have any more questions Ill reply as best I can.