Ziatora's Envoy

“If you don't” in the second ability refers to whether you played a land or cast a spell from the top of your library, not just whether or not you cast a spell.

You don't have to attack with the creature with blitz unless another ability says you do.

The triggered ability that lets its controller draw a card triggers when it dies for any reason, not just when you sacrifice it during the end step.

If a creature enters the battlefield as a copy of or becomes a copy of a creature whose blitz cost was paid, the copy won't have haste, won't be sacrificed, and its controller won't draw a card when it dies.

If you pay the blitz cost to cast a creature spell, that permanent will be sacrificed only if it's still on the battlefield when that triggered ability resolves. If it dies or goes to another zone before then, it will stay where it is.

If you choose to pay the blitz cost rather than the mana cost, you're still casting the spell. It goes on the stack and can be responded to and countered. You can cast a creature spell for its blitz cost only if you could cast that creature spell. Most of the time, this means during your main phase when the stack is empty.