Basic Properties of Electric Charge

The quantisation of electric charge, q = ne, applies to electric charge only, even though charge cannot exist without mass.

One simple rule to think here is that electric charge is a scalar quantity with magnitude. It has positive and negative signs, depending on the direction it is forced to move in an electric field. Mass is always positive. So when you add mass, it never cancels out or becomes zero. Also do consider that a charge can never exist when there is no mass. In calculations, you must remember not to take in the mass but just the charge itself. 

No, there may be some charges inside the system. The overall charge is zero because the total positive charge will cancel out the total negative charge inside. This directly follows the principle of additivity of charge, where the positive and negative charges are the algebraic sum of each other. 

Even though charge cannot exist without mass, we can only have the quantisation of electric charge using the equation, q = ne. We look at the number of point charge and the electron's charge. 

One rule to think here is that electric charge is a scalar quantity. It has magnitude. It has both positive and negative signs, based on the direction it is forced to move in an electric field. Mass is always positive, on the other hand. So when you add mass, it never becomes zero. 

No, the system can still have charges inside. The overall charge is zero only because the total positive charge will cancel out the total negative charge inside. This directly follows additivity of charge rule. The positive and negative charges are the algebraic sum of each other. 

When two or more individual charges are present in a system, the total charge will be an algebraic sum of all individual charges and not the vector sum. Therefore, an electric charge is considered as a scalar quantity.

This is an interesting question! Even though a proton has a positive charge, the net positive charge in a conducting material is always due to the removal of free electrons. 

This happens due to the availability of only free electrons in all conductors. Since protons are present in the nucleus, they can not roam freely in the conductor, as electrons travel freely in the conductor due to their presence in the outer shell.