Q1: Can I use this card's effect to target one of my opponent's Digimon with 4000 DP or less, reduce its DP to 0, and delete it?

Yes, you can.

Q2: This card attacks an opponent's Digimon, then that Digimon's DP is reduced to 0 and deleted by this card's effect. What happens to the attack?

No battle occurs, and the attack ends. If there are other effects to activate (When Attacking effects, etc.), the attack ends after activating them.

Q3: This card attacks an opponent's Digimon, then my opponent blocks with a Digimon that has Blocker. Can I use this card's effect to reduce the DP of the blocking Digimon to 0 and delete it?

No. Blocker and other effects that activate when an opponent's Digimon attacks only activate during the reaction timing window that occurs after When Attacking effects, etc. finish activating. As such, once a block has been declared, the window for activating this card's inherited effect has already passed, so it can't be activated.