Why shouldn’t your open the tank valve fully?
If someone wants to check if your tank is open, that person will possibly believe it’s closed and use a more force to open it. Afterward it’s very hard to close the valve again or it’s even broken.
Why does the second stage sometimes free flow at the surface?
This happens if the second stage hits the surface of the water with the mouthpiece pointing upwards. Based on the high density of water the purge button will be pushed.