An enemy who has a mark from Combat Challenge is adjacent to my fighter and attempts to charge a distant enemy.
Combat Challenge says that I can get an attack as an immediate interrupt whenever "an adjacent enemy attacks somebody else".
Will my fighter get the immediate interrupt attack even though the actual fight happens away from the fighter (after the enemy moves its speed during the charge)?
No.
You can take the opportunity attack when they leave the space adjacent to you, but the attack does not happen until they arrive. Charge, while one action, is two separate components, the move and the attack.
No.
The enemy is not adjacent to you while it's making the attack that the charge action tells him to make. Luckily, opponents also provoke attacks of opportunity when moving away from you and those happen right before they make that step, so you get one attack anyway.
