Rulings
Q1: Can "trash any 8 digivolution cards from your opponent's Digimon" in this card's When DigivolvingWhen Attacking effect trash a total of 8 digivolution cards from multiple opponent's Digimon?
Yes, it can.
Q2: If this Digimon attacks after DNA digivolving, does "if DNA digivolving, trash any 8 digivolution cards from your opponent's Digimon" in this card's When Attacking effect activate?
No, it doesn’t. If this effect is activated When Attacking, only "delete 1 of their Digimon with as many or fewer digivolution cards as this Digimon" activates.
Q3: Does "then, delete 1 of their Digimon with as many or fewer digivolution cards as this Digimon" in this card's When DigivolvingWhen Attacking effect activate even when not DNA digivolving?
Yes, it activates.
Q4: If my opponent's Digimon have less than 8 digivolution cards when this card's When DigivolvingWhen Attacking effect activates, does that mean that those digivolution cards can't be trashed?
No, as many as possible are trashed.
Data Sources
Lv.7 GraceNovamon EX5-073 SEC Mega | Vaccine | Galaxy [DNA Digivolve] [Apollomon]+[Dianamon]: Cost 0 <Security A. +1><Blocker> [When Digivolving] [When Attacking] If DNA digivolving, trash any 8 digivolution cards from your opponent's Digimon. Then, delete 1 of their Digimon with as many or fewer digivolution cards as this Digimon. [All Turns] When this Digimon would leave the battle area by an opponent's effect, by trashing 2 same-level cards in this Digimon's digivolution cards, prevent it from leaving.
Lv.7 GraceNovamon EX5-073 SEC Mega | Vaccine | Galaxy [DNA Digivolve] [Apollomon]+[Dianamon]: Cost 0 <Security A. +1><Blocker> [When Digivolving] [When Attacking] If DNA digivolving, trash any 8 digivolution cards from your opponent's Digimon. Then, delete 1 of their Digimon with as many or fewer digivolution cards as this Digimon. [All Turns] When this Digimon would leave the battle area by an opponent's effect, by trashing 2 same-level cards in this Digimon's digivolution cards, prevent it from leaving.
Lv.7 GraceNovamon EX5-073 SEC Mega | Vaccine | Galaxy [DNA Digivolve] [Apollomon]+[Dianamon]: Cost 0 <Security A. +1> <Blocker> [When Digivolving] [When Attacking] If DNA digivolving, trash any 8 digivolution cards from your opponent's Digimon. Then, delete 1 of their Digimon with as many or fewer digivolution cards as this Digimon. [All Turns] When this Digimon would leave the battle area by an opponent's effect, by trashing 2 same-level cards in this Digimon's digivolution cards, prevent it from leaving.
Lv.7 グレイスノヴァモン EX5-073 SEC 究極体 | ワクチン種 | 銀河型 〔ジョグレス〕「アポロモン」+「ディアナモン」:コスト0 指定のデジモン2体を重ね、アクティブで進化する ≪Sアタック+1≫≪ブロッカー≫ 【進化時】【アタック時】ジョグレス進化していたなら、相手のデジモンの進化元を選んで8枚破棄する。その後、進化元の枚数がこのデジモン以下の相手のデジモン1体を消滅させる。 【お互いのターン】このデジモンが相手の効果でバトルエリアを離れるとき、このデジモンの進化元から、Lv.の同じカード2枚を破棄することで、離れない。