Must a constant combined force on a curved motion be decomposable into a force along the direction of the velocity and a force perpendicular to the direction of the velocity, why

Yes. Because a force can be decomposed at will, as long as the two component forces satisfy the law of parallelograms with that force. An perpendicular to the direction of velocity and parallel to the direction of velocity decomposition is just one of many decompositions, but this one decomposition can help in understanding and doing problems, so we often decompose it this way.