A charger designed for a 12 volt battery should not be used to charge an 8.4V battery. It is likely to damage the battery and in the worst case can cause a fire.
There are some chargers that can be used with a variety of battery voltages and chemistries but unless you are certain that the charger is safe and effective on a different battery voltage, assume that it cannot be used.
Hard to say. It should have a decent charge in 6 hours if the battery is good.