Does lamictal also help with nerve pain? Take 100 mg daily for seizures but also have nerve pain in hand/wrist related to old brachial plexus injury

Doubtful benefit. Although on occasion, there is some pain benefit, usually far better effects with drugs such as gabapentin, lyrica, (pregabalin) cymbalta. But remember that if you have epilepsy, Lamictal is the first choice of most epilepsy specialists.
No effect on pain. Lamictal in general should have no effect on nerve pain, but may help nocioceptive pain by stabilizing mood.