Q1: If one of my Digimon digivolves after I use this card, does that Digimon still receive this card's effect?

Yes, it does.

Q2: After activating this card's effect, I use BT1-089 Mimi Tachikawa's effect to move a Digimon from my breeding area to my battle area. Does this card's effect also apply to that Digimon?

Yes, it does.

Q3: I attack an opponent's Digimon with a Digimon that's gained the When Attacking effect from this card. The When Attacking effect reduces my opponent's Digimon to 0 DP, deleting it. What happens next?

No battle occurs, and the attack ends. If there are other When Attacking effects, "when one of your Digimon attacks" effects, and/or effects activated at counter timing, they activate, and then the attack ends. If it's then blocked at blocker timing, start a battle with the blocking Digimon.

Q4: I play this card, then play a Digimon. Does it receive this card's effect?

Yes, it does.

Q5: I use 2 copies of this card. When I attack with a Digimon, can I activate its When Attacking effect twice in a single attack, giving an opponent's Digimon -4000 DP?

Yes, you can.

Q6: I have activated BT1-104 Golden Ripper and attack with BT3-058 BanchoStingmon. If I use this card's effect to decrease the attack of the attack target from 12000 DP to 10000 DP before I use BanchoStingmon's effect, can I give it +7000 DP?

Yes, you can still use BanchoStingmon's effect if the attack target's DP decreases before BanchoStingmon's effect activates as long as it was 12000 DP when it was targeted for the attack.