Over heating 93 BMW 525i

Sorry if this is too basic but I am doing a little research and I would greatly appreciate help as I know little of car repair.
If the car is leaking coolant and overheating what might the mechanic first test to be the problem? Would it be difficult to diagnose if it was the head gasket instead of the radiator or thermostat?
If the leak is stopped and the car continues to overheat and it is later determined to be a damaged head gasket would the head gasket damage be more likely due to poor repairs of the radiator and thermostat, or pre-existing damage from over heating?
If the car was running hot after repairing the cracked radiator would a technician ever tell a person to drive it anyway to knock out the air bubbles?
Would you expect a competent repair place to diagnose the problem quickly?

Again I apologize for these forms of questions with little detail but I would like to hear comments even if they are general and not conclusive. Thanks for your time.