There is no connection between the internal contentions of Zarahemla and the battle with the Lamanites, except that Mormon notes that "king Benjamin gathered together his armies." It may be that the internal contentions were temporarily resolved by the necessity of banding together against a more immediate external threat.
Mormon mentions that Benjamin fought "with the strength of his own arm" which suggests that Benjamin was active in the actual fighting. Mormon also notes that Benjamin employed the sword of Laban. By this time, the sword of Laban is over 300 years old, and a sacred/political relic. Although certainly wielded physically in the past, the reference to wielding the sword of Laban may here be more ceremonial than physical. The sword of Laban was a metal sword, and while durable, not imperishable. It would have been cared for, but 300 years of maintaining an edge would have diminished the strength of the blade, as swords are sharpened by removing small amounts of the metal and forming a new edge. It is most likely that wielding the sword was a visual symbol rather than a weapon at this point in Nephite history.