1. In my opinion he is in the manual labour category since he is operating and maintaining machinery.
2. OT must be paid in accordance with the EA;
3. I don't think you can use leave to substitute OT payment.
4. For employees not covered by the Employment Act, the employer, with the agreement of the employees, may fix any overtime rate he wants. But if you say you don't want to pay overtime at all, I don't think you can get the employees to work overtime for you.