Annihilator abilities trigger and resolve during the declare attackers step. The defending player chooses and sacrifices the required number of permanents before they declare blockers. Any creatures sacrificed this way won't be able to block.

If a creature with annihilator is attacking a planeswalker, and the defending player chooses to sacrifice that planeswalker, the attacking creature continues to attack. It may be blocked. If it isn't blocked, it simply won't deal combat damage to anything.

Casting Hand of Emrakul by paying its alternative cost doesn't change when you can cast it. You can cast it only at the normal time you could cast a creature spell.

Casting Hand of Emrakul by paying its alternative cost doesn't change its mana cost or mana value.

Effects that increase or reduce the cost to cast Hand of Emrakul will apply to it even if you choose to pay its alternative cost.