1) Does this mean ONE HEART per MONSTER or one for all MONSTERS?
One Heart per monster.
Example: If you roll 2 Hearts you can heal one Monster with both, or two Monsters for 1 each (and/or yourself of course).
3) Do Monster still pay if they are already had max health?
"...must pay you for each damage you heal..."
They only pay if they actually regain health.
4) Does the monster health still goes up if they dont have ENERGY to pay?
You can still heal others if they cannot pay. So it is in your best interest to not heal them.