After Cet possesses Kerah and disappears, the game crashes at a random point during my battle with her. Does anyone know when there will be a patch to fix this problem? The game cannot be finished until there is.

Do you have a saved game at the beginning of the fight, or within the fight? Many of us have found that using magic against Kerah at this point does crash the game, so try to keep your fight physical only, until she asks for a drink and you can give her your holy water (which you got earlier, naturally, filling those empty flasks).