If you force an enemy to attack through one of your powers, do they trigger an ally's
Combat Challenge fighter class feature ? ... assuming they were marked by the fighter
Yes they do.
Combat Challenge is an
Immediate Interrupt which means that as long as it is not the fighter's turn it can be triggered by any attack. It also does not specify that the enemy must be on its turn, or that it attacks willingly.
In addition, whenever a marked enemy that is adjacent to you shifts or makes an attack that does not include you, you can make a melee basic attack against that enemy as an immediate interrupt.
If the enemy gets to target one of the fighter's allies, or is forced to target one of its own allies it makes no difference, it is making an attack that does not include your fighter and therefore is a mark violation and subject to combat challenge. Bonus: it also take the
Mark penalty on that attack so its less likely to hit.