You might be able to start it by tapping starter with some metal rod as brushes in it got stuck from accumulated dirt and by tapping it they might get released and make contact again ,but you will have to refurbish it eventually .

depends on the vehicle but usually not. sometimes if voltage is an issue, yes you can, or if your starter is going away you can hit it with 24 volts to make it crank, but never would i try 24v on a modern computer system unless the car was junk.

id first rule out safety systems and security

Hi so adding more batteries is not going to start it i is the starter motor which is faulty.

no, a bad starter needs to be replaced. extra power will not make it better if the starter is indeed bad.

no that wouldnt work