Altaïr Ibn-La'Ahad

Although the tokens enter the battlefield attacking, they were never declared as attacking creatures. Abilities that trigger whenever a creature attacks won't trigger when they enter the battlefield attacking.

If an exiled card has {X} in its mana cost, X is 0.

Each of the tokens copies exactly what was printed on the card in exile and nothing else. They don't copy any information about the object the card was before it was exiled. 

You choose which player, planeswalker, or battle the tokens are attacking. Each token can enter attacking a different player, planeswalker, or battle, and they don't need to be the same one that Altaïr is attacking.

If a card copied by one of the tokens had any "when [this permanent] enters the battlefield" abilities, then the token also has those abilities and will trigger them when it's created. Similarly, any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities that any of the tokens have copied will also work.