No Errata
Q: If I use Fatal Riposte against an enemy fighter who is using an Attack action that targets multiple fighters, and it takes that enemy fighter out of action or drives them back so that they are not within range of their remaining targets, what happens to the rest of their Attack action?
A: As they can no longer resolve their action, it ends without being resolved any further.
Q: If I use Fatal Riposte and Steelheart has an Attack action that targets all adjacent fighters (e.g. his Mighty Swing) can I use that Attack action rather than an Attack action that only targets the attacker?
A: Yes, as long the attacker is one of the targets.