It will naturally lose some of its power, but should still be hot enough to start the vehicle if it's a good battery, and was fully charged when stored.
This will depend on the size of the 12 v battery.
Yes
If you jump a car or other vehicle with a 6 volt electrical system using a 12 volt battery, you can blow out the 6 volt battery. When they are connected, it usually only takes a fraction of a second to start the six volt system. There should be a person ready to start the 6 volt system immediately if not sooner so the 12 volt system can be disconnected immediately before the battery overheats and burns out.
I would not buy any battery over 6 months old.
Check the voltage at the battery with a DC volt meter. It is reads 12.6 volts is it 100% charged. 12.4 volts 75%, 12.2 volts 50%, and 12.0 volts 25% charged. Less than 12.0 volts is a dead battery. If the battery will not hold a charge with the negative cable disconnected it has a dead cell and must be replaced.
No
if it is any think over 1 volt it is not a cell battery it is just a 6 volt battery if you had a 1 volt then it would be a cell battery
No, it will overcharge the 6 volt battery.
The battery is a 12 volt.
No
No, it will not shut off. Overcharging because the voltage regulator is defective.
No, you cannot charge an 8 volt battery with a 6 volt charger. The voltage of the charger must match the voltage of the battery.