You may cast Portal Manipulator outside of a declare attackers step. If Portal Manipulator enters the battlefield outside of a declare attackers step, its ability simply doesn't trigger.

Portal Manipulator's triggered ability ignores all requirements, restrictions, and costs associated with attacking.

Portal Manipulator's triggered ability doesn't cause "whenever this creature attacks" abilities to trigger.

Once Portal Manipulator's triggered ability resolves, the target attacking creatures are still considered to have attacked the player, planeswalker, or battle they were initially declared as attacking, but they are now attacking the target player.

If an ability targets something controlled by the "defending player" of an attacking creature and the defending player for that creature changes before that ability resolves, the ability won't resolve because its target has become illegal.