Correct, he gets both.
Resolve them in whatever order you see fit, but it doesn't matter too much--they're effectively simultaneous. Note however that combat challenge attacks do not stop movement. Only opportunity attacks do that. Regardless, both attacks would go off; you don't cancel an interrupt because the triggering action was invalidated.
Right, I got that backwards. So the monster starts to move toward the warlord, so in response, the fighter gets his Combat Challenge attack, then he also immeadiately gets an OA due to the warlord's Viper's Strike. This attack is an OA so it does stop the monster's movement, meaning the monster must now use another move action to close on the warlord and not getting an attack at all.
Is this correct? It seems very powerful and effectively shuts down a lot of monsters (especially solos) or forces them to attack the fighter no matter how ineffective that might be considering the possiblity of healer keeping up with any damage done by the monster.