3/4 full is apparently the sweet spot, per the Goldilocks Effect, says my best googling.
Correct. It needs to have air to circulate.
If you have an empty refrigerator and let it run until it stops, it's full of cold air. The moment you open the door, most of the cold air "falls out" and is replaced by warm air, so it has to run an extended amount of time to cool the air again.
When it's full, opening the door has a minimal effect on the temperature since everything acts like a cold "sink" and it only has to run a short period of time to cool off the little bit of air that entered.
In a 30 day period, it can have a substantial effect on your electric bill.