

Q1: My opponent has a level 6 or lower Digimon. Can I meet the "if this effect didn't delete" condition in this card's All Turns effect by not choosing a card to delete?
No, you can't. If your opponent has a level 6 or lower Digimon, you must choose that card and delete it.
Q2: My opponent has a level 6 or lower Digimon. Can I meet the "if this effect didn't delete" condition in this card's All Turns effect by choosing an opponent's level 6 or lower Digimon with an effect that prevents deletion?
Yes, you can.
Q3: What does "end the attack" mean, exactly?
After this effect activates, the current timing makes a transition to the end of attack timing. For example, if this effect activates during the attack declaration timing, it will make a transition to the end of attack timing. A transition to the counter timing or block timing won't occur, and the attack won't succeed.
Q4: Can an "end the attack" effect end an attack by a Digimon that isn't affected by effects?
Yes, such attacks can be ended. "End the attack" effects are effects that change the timing, they don't affect an attacking Digimon.
Lv.6 Necromon BT23-069 R Mega | Virus | Ghost/LIBERATOR <Execute> [On Play] [On Deletion] You may play 1 level 5 or lower Digimon card with the [Ghost] trait from your trash without paying the cost. [All Turns] When another Digimon attacks, by deleting this Digimon, delete 1 of your opponent's level 6 or lower Digimon. If this effect didn't delete your opponent's Digimon, you may end that attack. ---
Lv.6 Necromon BT23-069 R Mega | Virus | Ghost/LIBERATOR <Execute> [On Play] [On Deletion] You may play 1 level 5 or lower Digimon card with the [Ghost] trait from your trash without paying the cost. [All Turns] When another Digimon attacks, by deleting this Digimon, delete 1 of your opponent's level 6 or lower Digimon. If this effect didn't delete your opponent's Digimon, you may end that attack. --- -
Lv.6 Necromon BT23-069 R mega | Virus | Ghost/LIBERATOR <Execute> [On Play] [On Deletion] You may play 1 level 5 or lower Digimon card with the [Ghost] trait from your trash without paying the cost. [All Turns] When another Digimon attacks, by deleting this Digimon, delete 1 of your opponent's level 6 or lower Digimon. If this effect didn't delete your opponent's Digimon, you may end that attack.
Lv.6 ネクロモン BT23-069 R 究極体 | ウィルス種 | ゴースト型/リベレイター ≪エグゼキュート≫ 【登場時】【消滅時】自分のトラッシュから、特徴に「ゴースト型」を持つLv.5以下のデジモンカード1枚をコストを支払わずに登場できる。 【お互いのターン】他のデジモンがアタックしたとき、このデジモンを消滅させることで、Lv.6以下の相手のデジモン1体を消滅させる。この効果で相手のデジモンが消滅しなかったなら、そのアタックを終了できる。 ---