

Q1: If I activate this card's All Turns effect when multiples of my other Digimon with the Reptile or Dragonkin trait would leave the battle area at the same time by my opponent's effects, are all of those Digimon prevented from leaving?
Yes, all of those Digimon are prevented from leaving. This card's All Turns effect affects all Digimon without having to choose them.
Q2: In what order do a Security effect, "when ... performs a security check" effect, and "when a card is removed from ... security stack" effect activate when they trigger simultaneously upon a security check?
Security effects take precedence for activation. Upon a security check, a Security effect will immediately activate without pending activation. For other triggered effects, the turn player activates their effects first.
Lv.4 Dimetromon BT24-012 U Champion | Virus | Reptile/LIBERATOR <Blocker> [All Turns] When any of your other Digimon with the [Reptile] or [Dragonkin] trait would leave the battle area by your opponent's effects, by returning this Digimon to the hand, they don't leave. --- [Your Turn] [Once Per Turn] When your opponent's security stack is removed from, gain 1 memory.
Lv.4 Dimetromon BT24-012 U Champion | Virus | Reptile/LIBERATOR <Blocker> [All Turns] When any of your other Digimon with the [Reptile] or [Dragonkin] trait would leave the battle area by your opponent's effects, by returning this Digimon to the hand, they don't leave. --- [Your Turn] [Once Per Turn] When your opponent's security stack is removed from, gain 1 memory.
Lv.4 Dimetromon BT24-012 U champion | Virus | Reptile/LIBERATOR <Blocker> [All Turns] When any of your other Digimon with the [Reptile] or [Dragonkin] trait would leave the battle area by your opponent's effects, by returning this Digimon to the hand, they don't leave. --- [Your Turn] [Once Per Turn] When your opponent's security stack is removed from, gain 1 memory.
Lv.4 ディメトロモン BT24-012 U 成熟期 | ウィルス種 | 爬虫類型/リベレイター ≪ブロッカー≫ 【お互いのターン】特徴に「爬虫類型」/「竜人型」を持つ他の自分のデジモンが相手の効果でバトルエリアを離れるとき、このデジモンを手札に戻すことで、離れない。 --- 【自分のターン】[ターンに1回]相手のセキュリティが減ったとき、メモリー+1。