But when did they even learn balefire? It was not until the War of the Shadow that they even realized what it did, and that War did not fully break out until the last ten years before the Breaking. Meanwhile, a man who is still single on the day of the Bore has a grown grandson on the day the war ended. By that reckonning, decades past before anyone could possibly have known enough about balefire to go after Mierin/Lanfear (and by that point she would have been sufficiently infamous that she'd have already been a high-priority target and almost impossible to get to, or someone would have whacked her long before then), and Beidomon was already likely dead.