What is the ruling on paying zakat on money and the monthly returns on sums that have completed a hawl (lunar year), and is this approach Islamically permissible? And what is the ruling on sums withdrawn before a hawl has passed on them?

The method you have followed to pay zakat is correct for the new money you acquire, such as salary and gifts. You can find an easier method for calculating zakat on salary in answer no. 30316. As for the profits from investment funds, their hawl (zakat year) is the same as the hawl of their principal, and all of it is subject to zakat upon the completion of the hawl of the principal. There is no zakat on amounts withdrawn during the hawl. The hawl considered for zakat is the lunar hawl, as mentioned in answer no. 30364.