The effect forces that specified spirit to block. The specified spirit is immediately declared as a blocker, even if it is exhausted.

Attacking a specified spirit forces it to be declared as a blocker. Therefor, one: the attack cannot reduce the life of your opponent. Two: the flash window between attack declaration and block declaration is skipped since a blocker has been declared. The remainder of the battle is then managed normally.

No. No other spirits can block the attack.

That spirit is immediately declared as a blocker and its When This Spirit Blocks effect activates directly after the effect responsible for the attack.

No, they cannot. Since the attack forces that spirit to be declared as a blocker, the flash window between attack declaration and block declaration is skipped.

No, it cannot. Since effects preventing some action take precedence, the block fails to occur when a spirit which cannot block is selected, and the rest of the battle proceeds normally.

The block was established when the effect responsible for the attack resolved, so no flash window prior to block declaration occurs, and no life is lost from applying damage.