Maybe - it depends on what device will be used. Many devices that run off a car battery will have some kind of voltage conversion and regulation so often times it will work just fine (at the expense of heat generation)- but your mileage will vary as it depends on the specific device.
No
No, it will overcharge the 6 volt battery.
Yes, that is acceptable.
No it will not work
yes it does. i used a 8.4 in the same gun and it works but don't use a 9.6 or your screwed.
No, the battery will be overcharged and battery damage will occur if it is done.
No
No.
can 6 ,12 volt battery be wire to make in to one 24 volt battery for solar use in home
12 volt battery
No.
Absolutely not! It is a 14 volt drill. Use a 16 or 18 volt battery will destroy the drill in short order. It will run but not for long.