Regarding hold temperatures .....
All baggage holds are pressurised the same as the cabin.
Most of the holds are not heated. They don't need to be as experience shows that the ambient temperature when the bags are loaded is maintained, more or less, for the duration of the flight. I've always meant to check this by putting a minimum/maximum thermometer in my baggage, but am assured by an aircraft engineer that the temperature does not drop below freezing.
One of the holds is heated on most aircraft, and that is where any particularly temperature-sensitive freight, and animals in cages, are placed. It's a chance if your baggage goes in that one.