are men and women allowed to sit together during benching, and if so why is this allowed in the face of having to daven with a seperation?

A small or close family can sit together. More distant relatives or friends should not sit together during a meal and benching. During davening one needs a separation in order that there be Kedusha. Kedusha does not appear in a place of mixure of men and women. Therefore all agree that there need be a Mechitza during prayer. Rabbi Yitzchak Grinblat