Q1: I played a Digimon with 5000 DP or less in a state where my opponent used this card's When DigivolvingOn Deletion effect to give all of my Digimon -5000 DP. Can I activate my played Digimon's On Play effect before it's deleted by its DP becoming 0?
No, you can't. If a card is played at the same time as deletion by DP becoming 0, the On Play effect is triggered, but it can't activate because the card is deleted before it activates.
Q2: Do I have to activate this card's End of Attack effect?
Yes, you must activate it whenever possible.
Q3: Does this card's End of Attack effect also activate when my opponent has no Digimon in play?
Yes, it does.
Lv.7 ShineGreymon: Ruin Mode EX4-074 SEC Mega | Vaccine | Light Dragon Digivolve: 4 from [ShineGreymon] [When Digivolving] [On Deletion] Until the end of your opponent's next turn, all of your opponent's Digimon get -5000DP. [End of Attack] Delete this Digimon and 1 of your opponent's Digimon, and <Recovery +1 (Deck)>. Then, if you have a Tamer in play, hatch 1 Digi-Egg card to an empty space in your breeding area.
Lv.7 ShineGreymon: Ruin Mode EX4-074 SEC Mega | Vaccine | Light Dragon Digivolve: 4 from [ShineGreymon] [When Digivolving] [On Deletion] Until the end of your opponent's next turn, all of your opponent's Digimon get -5000DP. [End of Attack] Delete this Digimon and 1 of your opponent's Digimon, and <Recovery +1 (Deck)>. Then, if you have a Tamer in play, hatch 1 Digi-Egg card to an empty space in your breeding area.
Lv.7 ShineGreymon: Ruin Mode EX4-074 SEC Mega | Vaccine | Light Dragon {{DigivolveFromNames|4|ShineGreymon}} [When Digivolving] [On Deletion] Until the end of your opponent's next turn, all of your opponent's Digimon get -5000DP. [End of Attack] Delete this Digimon and 1 of your opponent's Digimon, and <Recovery +1 (Deck)>. Then, if you have a Tamer in play, hatch 1 Digi-Egg card to an empty space in your breeding area.
Lv.7 シャイングレイモン:ルインモード EX4-074 SEC 究極体 | ワクチン種 | 光竜型 【進化時】【消滅時】次の相手のターン終了まで、相手のデジモン全てをDP-5000。 【アタック終了時】このデジモンと相手のデジモン1体を消滅させ、≪リカバリー+1《デッキ》≫。その後、自分のテイマーがいるなら、空いている自分の育成エリアにデジタマカード1枚を孵化させる。