The 24th and final Piece of Heart, coincidentally, is located on Death Mountain -- and you can only access it via the Turtle Rock dungeon, so it's best to pick it up when finishing up the final ...

Yes, it is possible to save Fior mes Ivèrno from burning, although it will still be attacked if you take the steps to do so. Basically, the city guard will be able to fight off the Steel Garrote ...