Real Iron Palm training should only be done under careful supervision by someone who has some serious experience and qualifications. You can **** yourself up big time and permanently by doing it wrong. Traumatic arthritis is no joke. Think about it. You're not the Emperor's Bodyguard who has nothing to live for but being a Living Weapon. You're a 21st century city-dweller who needs those hands for typing, playing the piano and tying your children's shoe laces.
Anyway, concerning palm heel strikes, they have definite advantages. I'd much rather hit someone in the head with one of those than with my fist. You don't lose that much range, and if you're punching correctly there will be plenty of power. If you're arm-punching you need to go back to class and practice more
Leaving the fingers apart is just asking to get one broken. I find I have much a much more relaxed strike with less antagonism between the muscles when the fingers are curled. If you look at the classics like Fairbairn and Applegate you'll see that they taught it pretty much that way. Their record in the real world speaks for itself.