I just checked, Harry indeed can get the Dragon Nail needed for Health potions (I got level 8, not 9 or 9).
Is that a change once you reach FoV part of the game ?
I'm pretty sure I couldn't get those in ED part (I don't have a game in ED where I already have the tower, so i can't check right now).
By once Ygerna is killed, I meant once she is burnt, so just before we get to duel her (to destroy her soul, I guess). It's the how we get back to the prison world. The other times Bellegar teleports you right in, but this time there is no cutscene or anything like that.