To the best of my knowledge, no valve manufacturer makes a double seat low pressure valve. Keep in mind, it takes very little leakage to sound like a hole in the boat.
If you feel more comfortable opening it all the way, I say go nuts! There is no gains as far as performance of the tool to be had by 1/2 turn or 12 turns.
As far as leaks in the stem and bonnet, if there is one, the supply house should be notified and the valve should be replaced.
The curiosity is killing me. what manufacturer manual do you have that shows a double seat? I would be interested into looking at one.
Thanks for your input on this Oldred.