Hello, I hope I can offer you some assistance.
The DRIVe button is to enable or disable the start/stop function of the car. There are many factors that affect the start/stop function but the main thing is battery charge and condition. As the car has been stood at a garage for a while then the battery charge is probably too low for the start/stop to function, the battery state of charge should be at least 72% for the start/stop to function. Other factors for the system to operate are the vehicle needs to be in neutral, the driver's seat belt fastened the driver's door closed and handbrake on. There are other factors as well such as air temperature, engine temperature electrical load and others.
The best thing to do if you're unsure is to take the car for a very long drive of at least 50 miles, this way the battery monitoring checks the system correctly and will detect the increase in battery charge state, you can't just charge the battery with a charger as if the battery is off the car the battery monitor will stay at the same when you reconnect the battery, it can't detect the charge added by the battery charger.
The fact you stalled the car but then it restarted when you pressed the clutch would suggest the system is functioning correctly but the battery is too low for the system to stop the engine ( the system wants to be 100% certain the engine will restart).
I hope this clarifies things for you but if I can assist further let me know and I'll reply as soon as I'm able to.
Best wishes and happy new year.