No. The points you pay for a chariot include its crew, whether that chariot is included in your army on its own or as a mount for a character. However, in reality it might prove difficult to fit a character and the full complement of crew into the chariot model itself. If so, it is perfectly acceptable to remove one or more of the crew models to make space for the character. In such cases, we can think of the missing crew model's profile(s) as a way of representing that the beasts pulling a character's chariot are bound to be bigger, stronger and better trained than normal.

No. You must use a magic item fully or not at all.

No, a shield is carried by the character, not the mount.

Yes.

No.

The purpose of the Targeting Lone Characters rule is to protect characters from enemy shooting, even from shooting that does not follow the usual rules (though it does not protect them from templates). Therefore, and in the spirit of this rule, unless they are the closest target to the cannon, an enemy character that is within 3" of a friendly unit that contains five or more models of the same troop type cannot be struck by a cannonball, even if their base lies directly under the path of the bouncing cannonball. However, if a Lone character's base lies directly underneath the strike point of a cannonball, they are hit.

No.

They must use their magic weapon. However, some named characters will carry a 'back-up' weapon which they can use should their magic weapon be destroyed.