No it cannot.
Install a 3 volt battery and forget any modifications. It will not operate on 12 volts.
No, you will need two 12 volt batteries connected in series to give the 24 volts necessary to run the pump. It will not run on 12 volts.
No
The capacities of batteries are rated in amp hours. The higher the amp hour rating of a battery the longer the device connected to the battery will operate. Depending on what the connected load draw in amps is, will determine how long the connected device will operate.
Charge it with a 12 volt battery charger.
No
12 volt
A 12 Volt deep cycle marine battery.
No, a 12 volt charger will overcharge a 8 volt battery and destroy it.
The battery is a 12 volt.
No
Yes, a 12 volt battery will be damaged if charged with a 24 volt charger.