Will even apply to creatures that entered the battlefield after it resolved (which obviously must have Haste in order to attack).

If a creature is attacking a planeswalker, assigning its damage as though it weren't blocked means the damage is assigned to the planeswalker, not to the defending player.

When Predatory Focus resolves, you choose whether to use its effect or not. If you choose to use it, all your creatures will deal their combat damage to the planeswalker or defending player this turn whether or not they become blocked. You can't have any of them deal combat damage to creatures that block them. If you choose not to use its effect, nothing happens.