

Q1: If there are 2 face-down cards then 1 face-up card in order from top to bottom in this card's digivolution cards, what card do I trash for this card's When Digivolving When Attacking effect?
You trash the 1st face-down digivolution card from the bottom.
Q2: 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.5 Etemon EX9-031 R Ultimate | Virus | Puppet/DM/Ver.3 When any of your [Ver.3] trait Digimon would digivolve into this card, for each of their face-down digivolution cards, reduce the digivolution cost by 1. <Security A. +1> [When Digivolving] [When Attacking] [Once Per Turn] By trashing this Digimon's bottom face-down digivolution card, <Recovery +1 ≪Deck≫>. --- [All Turns] [Once Per Turn] When your security stack is removed from, 1 of your opponent's Digimon gets -4000 DP for the turn.
Lv.5 Etemon EX9-031 R Ultimate | Virus | Puppet/DM/Ver.3 [Digivolve] [Sukamon]: Cost 3/Lv.4 w/[DM] trait: Cost 4 When any of your [Ver.3] trait Digimon would digivolve into this card, for each of their face-down digivolution cards, reduce the digivolution cost by 1. <Security A. +1> [When Digivolving] [When Attacking] [Once Per Turn] By trashing this Digimon's bottom face-down digivolution card, <Recovery +1 ≪Deck≫>. --- [All Turns] [Once Per Turn] When your security stack is removed from, 1 of your opponent's Digimon gets -4000 DP for the turn.
Lv.5 Etemon EX9-031 R ultimate | Virus | Puppet/DM/Ver.3 [Digivolve] [Sukamon]: Cost 3 [Digivolve] Lv.4 w/[DM] trait: Cost 4 When any of your [Ver.3] trait Digimon would digivolve into this card, for each of their face-down digivolution cards, reduce the digivolution cost by 1. <Security A. +1> [When Digivolving] [When Attacking] [Once Per Turn] By trashing this Digimon's bottom face-down digivolution card, <Recovery +1 (Deck)>. --- [All Turns] [Once Per Turn] When your security stack is removed from, 1 of your opponent's Digimon gets -4000 DP for the turn.
Lv.5 エテモン EX9-031 R 完全体 | ウィルス種 | パペット型/DM/Ver.3 特徴に「Ver.3」を持つ自分のデジモンがこのカードに進化するとき、そのデジモンの裏向きの進化元1枚ごとに、支払う進化コスト-1。 ≪Sアタック+1≫ 【進化時】【アタック時】[ターンに1回]このデジモンの裏向きの進化元を下から1枚破棄することで、≪リカバリー+1《デッキ》≫。 --- 【お互いのターン】[ターンに1回]自分のセキュリティが減ったとき、ターン終了まで、相手のデジモン1体をDP-4000。