It's orange charging light will turn off when finished.
Yes
Yes. When i was charging it, I turned off my Xbox and the lights still indicated it was charging.
An electrical device can still be on and charging if the fan is off. If the device is plugged in to the wall, or connected to a battery, chances are it is charging.
You HAVE to turn it off when charging, so yeah, you can :)
you dont.
Hydrogen
No.
Generally, automotive batteries discharge Hydrogen Gas during both the charging and discharge cycles.
Keep it turned off while charging.
Have the charging system checked. The light comes on when there is a charging system problem. it may be a battery, alternator or a battery problem.
While your tablet is charging it is best to turn it off charging if you want it to charge quicker. Playing apps can take triple the time it should do to charge. If you do not need it charged fast have fun with it. It doesn't really matter if you've got lots of time :)