[@Conch Shell VII] The Elemental King would have been ruling for a long time. So the human population would of gone down a lot. However there would be children who would get past his eyes. Which is how the Elemental Chosen Ones came to be. Not only that, he wouldn't allow the human population to die out and would enslave children and raise himself, only killing them if they showed any promise.

And for healing light, think of it as magic. You would gather holy light in your hand and aim it towards the injury. It would then heal the wounds using light powers, purifying any infections while doing so. Another way of looking at is speeding up the recovery time using Light. There are many explanations but ultimately it's up to you.