Caldaia Guardian

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 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.