I suspect Leah's father to be Lachdanan. Im sure im not the only one with that speculation.
http://diablo2.diablowiki.net/Adria_the_Witch#Gossip ,heres a gossip from Adria when you ask her about lachdanan (once you have the quest at lvl 14 into the cathedral)
"You may meet people who are trapped within the Labyrinth, such as Lachdanan.
I sense in him honor and great guilt. Aid him, and you aid all of Tristram"
all of tristram she sais, a mystic way to incluse herself. As a powerful wich, im sure she is able to keep many secrets. Probably one of them being lachdanan's love.
again its all speculation. i sense leah has a dark futur ahead...