it gets eaten by a tiny monster inside the battery which gives energy to the appliance
You will burn up your appliance!!!!!
No. The voltage of the charger's output is only 7.5 volts. This is not high enough to charge a 9 volt battery device.
No. The person who designed your appliance had a 12v adapter, so when he designed the adapter connector he put a resistor in it to drop the voltage to 9v.
Covering a battery with a lemon will ruin the battery and will not make the appliance to which the battery is connected go faster.
You haven't mentioned the voltage or the Amp-hour capacity of the battery. In order to estimate the life of the battery, we need to know both of those, and we also need to be assured that the battery voltage is the same as the designed operating voltage of the appliance.
If there happens to be a fault on that appliance the ground wire is the faults path back to ground.
Use the one specified in the instructions, or a direct equivalent.
The minerals in the water will react with the acid and become compounds that will affect the life and capacity of the battery.
The flashlight can not be turned on as it no power now.
Most camper type vehicles have 2 batteries, one for vehicle operation and the other for appliance operation. The appliance battery should be "isolated" from the other, meaning that it should only charge when the engine is running and should not draw power from the other battery. Battery isolation circuits are available at most RV supply stores.
There are many places where you can buy used appliance parts. Many local restoration stores sell used appliance parts. You can also buy them online at Ebay (http://www.ebay.com) and Appliance Depot (http://www.appliancedepot.com).
The battery will explode.