Fnac takes a small sample from the swelling and examines it under the microscope. But in excision biopsy , we take out the swelling , cut it into small slices and examine each and every part of it under microscope . So, even if we have missed something in fnac, biopsy is the confirmatory. There are several reasons for swelling in posterior triangle. It can be a general infection like dandruff. Excision biopsy helps to find out whether we are missing any malignancy or even tuberculosis .

No! Reactive hyperplasia implies infective etiology. Since this lymphnode is in the posterior triangle of neck, it drains your scalp region which will have the source of the infection.

Considering your age 17 I feel it's nothing to worry. But I need more detailed information about your health concern. Excision biopsy is advised because FNAC was inconclusive.