Dear malikpmailk
1. Yes, management can make new employee comes to work even if the old employee is still around, serving his/her notice period. This practice however has both pros and cons. Advantages can include a proper handover and some form of training from old to new employees. Disadvantage as old employee (which may be a disguntled employee) may "poison" new employee. Please weigh the situation first before deciding course of action.
2. All sick leave must be supported by official medical leave certificate by appointed company doctors, if an employee has complied with sick leave policy and procedures (i.e. get medical chit from HR to visit doctor, informed HR/own department/own superior within stipulated timeline about sick leave and submit official medical leave certificate to HR upon returning back to work), Management/HR CANNOT report it as leave without pay, even when there is still balance in his/her annual leave entitlement.
Regards
Autumn Jane