An employee must be allowed to carry over any unused part of her statutory leave entitlement of 28 days (including bank holidays).
No, they must not take annual leave and maternity leave at the same time. If they are on annual leave when their baby is born, maternity leave will start on that day and their annual leave will finish.
Yes, if you agree. It may help an employee to settle back into work if this leave pattern can be accommodated however you will want to ensure that it is consistent with your business needs.
Further guidance about maternity leave, annual leave and sick pay is available on the NI Business Info's website