22 Jun 2015
You choose one card name, not one name per opponent.
22 Jun 2015
Alhammarret's second ability happens as Alhammarret enters the battlefield. No one can cast spells or activate abilities between the time a card is named and the time that Alhammarret's last ability starts to work.
22 Jun 2015
If your opponents have no nonland cards in their hands, you can't choose a card name. Alhammarret's last ability won't stop any spells from being cast in that case.
22 Jun 2015
Your opponents can still cast a card with the chosen name face down if it has a morph ability. A card with the chosen name can also be manifested.
22 Jun 2015
Although spells with the chosen name can't be cast, permanent cards with that name can still be put onto the battlefield by a spell or ability.