1. Can any Space Marine Army field him as a HQ choice?
Yes.

2. Under the ability: Challenge, if the emperor's champion wins the challenge, does the entire enemy squad fall back or do they get their hits at the champion?
No, the combat ends and they take a Moral Check as nornal.

3. Can he use challenge even if it was the opponent who was charging?
As far as I know Yes.

4. If there is more than one squad in CC with the emperor's champion, do they fight normally or do they have to watch the champion challenge?
Not sure.

5. Does his iron halo count towards the number of iron halos allowed in the army?
Yes.

 

if in a challenge other models cant attack the champian or the model being challenged they just sit and wait for the result

 

and most likly with their always hit on a 3+ e.c is most likly to kick some behind :lol:

 

If an Champion challenges someone then the two are seperated from the combat and fight only each other...the result of this one on one is the result of the combat as a whole, however all other models still get their attacks against each other so technically the combat is split into two....The champion against his 'victim' and the Champions squad against the other guys squad

Hope this helps... :blink: