Could you not simply make a zone for a certain part of the map and in that zone, make a script so that you cannot take any damage?
No, Not really at least. You could constantly give players back their max health, But if they happened to take enough damage to instantly kill them they would still die.
I can definitely see a use for a zone option that is basically the combat toggle in the game already. But for difficulty, As far as I am aware, The only difference between easy and normal is that on normal Dryads and Djinn will shoot explosive arrows at the player. Assuming this is the case, The only difficulty that really affects difficulty is legendary.