Souls have to be actually "sent" to Hell, they don't just go there on their own. The in-game universe treats Heaven and Hell as physical locations within Creation, so... you'd probably need physical anchor to carry you, a still living body to walk in the front gates, or somebody holding open a portal and channeling your spirit through with some weird brand of magic.
And as far as we can tell, while souls can be corrupted and bound to a location, they don't seem to follow the demon lords through the cycle of rebirth in the Black Abyss.
Also, if memory serves me right, my Witch Doctor said he didn't sense Leah enter the unformed lands.
Saying that, I would guess that Leah is still in the Black Soulstone. I'd also hazard that all the work Blizzard has done in building up how "important" Leah's lineage is, and how special her powers are, she likely isn't what we would consider an average human.
If Nephalem are half-angel and half-demon, and her parents are Aidan and Adria, there is a good chance that Blizzard will consider her to be more demon than the rest of mankind. I'm not really sure if Aidan is considered her father, or Diablo is... but Adria could definitely be a demon in disguise.
That would mean that while mortals have different rules as to what happens to them when they die, Leah might actually be able to survive beyond the destruction of her body, and come back as the complement of Tyrael.
He is an angel, but not. And likewise, she would be a demon... but not. Both have aspects that make them like their respective races, yet both are mortal and not "bound" to the natures of those races.
Toss in the Nephalem who are the true middle ground between the two races, we have an awesome powerhouse of characters who could turn Creation on its ear and completely mess up the status quo of the universe.
Regardless, unless we actually kill her, and we know that what we killed is actually Leah to her core (corrupted or otherwise), then I'm inclined not to believe that the girl is dead.