Kardur's first ability affects all creatures your opponents control, including any that enter the battlefield after the ability resolves.

If a creature an opponent controls can't attack for some reason (such as being tapped or having come under its controller's control that turn), then it doesn't attack. If there's a cost associated with having it attack, its controller isn't forced to pay that cost, so the creature doesn't have to attack in that case either.

If a creature an opponent controls doesn't qualify for any of the above exceptions and can attack, it must attack a player other than you if able. If the creature can't attack any of those players, or if there are no such players, it must attack you or an opposing planeswalker.

If a creature affected by Kardur's ability could legally attack multiple players and/or planeswalkers, its controller chooses who it attacks.

Kardur's last ability will trigger no matter who controlled the attacking creature and no matter who it was attacking.