Electrons moving in magnetic field have circular paths because the magnetic force is always perpendicular to the speed of electron. If the electron entering the magnetic field has an initial speed that it is not perpendicular to the magnetic field it will have an helicoidal trajectory.
1. Since the magnetic force is always perpendicular to the electron, the acceleration does not modify the speed in its absolute value. Therefore there are no relativistic effects related to changes in the absolute value of the speed, hence the mass remains the same.
2. As discussed above, the absolute value of the speed does not change. Also the mass is constant. Hence the kinetic energy remains the same.
3. The velocity of a particle is a vector. Since the trajectory of the electron is circular, even if the absolute value of the velocity does not change, its direction changes. Therefore the velocity of the electron changes.
3. Speed is a scalar, and with only a few exceptions, it is equal to the absolute value of velocity. As shown above, the speed of the electron does not change.