Mogg will deal 18 damage, but it will deal it after dying.
The en-Kor's redirection only works if you activate it before they take damage. When you activate their redirection ability, it creates a shield that will redirect the next point of damage it would take.
The redirection shield is a replacement effect. It causes Inferno to deal damage to Mogg instead of the en-Kor. Inferno still deals all of its damage simultaneously.
This is how the game progresses:
- You cast Inferno (and retain priority).
- Six times,
- You activate Shaman en-Kor's first ability targeting Mogg.
- Shaman en-Kor's first ability resolves, creating a shield that will redirect damage later.
- Six times,
- You activate Nomads en-Kor's ability targeting Mogg.
- Nomads en-Kor's ability resolves, creating a shield that will redirect damage later.
- Inferno resolves.
- It will simultaneously deal 18 damage to Mogg and 6 damage to each player. Simultaneously, life is subtracted from players' totals, damage is marked on Mogg and Mogg's ability triggers.
- State-based actions are performed simultaneously.
- Any player with non-positive life lose the game. (I'm going to assume noone loses here.)
- Mogg is destroyed for having marked damage exceeding its toughness.
- Mogg is put in the graveyard from the battlefield.
- Mogg's triggered ability is placed on the stack targeting your opponent.
- Mogg's triggered ability resolves.
- It deals 18 damage to the targeted opponent.
Note that Mogg only takes damage once in this scenario (even though some of it was originally intended for the en-Kor), so its ability only triggers once for the entire 18 damage.