No Errata
Q: What happens when I use Daemonic Weapon with the ploy Mighty Swing, targeting more than one enemy fighter?
A: The fighter suffers 1 damage before making each Attack action, so they would suffer 1 damage, then (assuming they survive) make their first Attack action, then suffer 1 more damage and then (assuming they survive) make their second Attack action and so on. If they are taken out of action, you cannot resolve the Attack action against any remaining targets.
Q: Can the damage my fighter suffers from using Daemonic Weapon trigger My Turn, and if so in what order is it resolved?
A: Yes. You would play My Turn after resolving the Daemonic Weapon Attack action.
Q: When does the fighter suffer the wound from using Daemonic Weapon?
A: The wound is suffered before the Attack action.
Q: Who has dealt the 1 damage suffered by a fighter making an Attack action with the Daemonic Weapon upgrade?
A: That fighter.