Q1: "My opponent's Digimon with Security Attack +1 attacks me and 1 card is removed from my security stack by the security check. If I use this card's Opponent's Turn effect to {{DeDigivolve|X}} that opponent's Digimon and it loses Security Attack +1, is the 2nd security check performed?"
No. If this card's effect causes Security Attack +1 to be lost before the 2nd security check, it isn't performed.
Lv.6 ShadowSeraphimon EX4-050 R Mega | Virus | Seraph Digivolve: 1 from [Seraphimon] [Opponent's Turn] When a card is removed from your security stack, <De-Digivolve 1> 1 of your opponent's Digimon. [On Deletion] <Recovery +1 (Deck)>. (Place the top card of your deck on top of your security stack.) Then, 1 of your opponent's Digimon gets -4000 DP for the turn for each card in your security stack.
Lv.6 ShadowSeraphimon EX4-050 R Mega | Virus | Seraph Digivolve: 1 from [Seraphimon] [Opponent's Turn] When a card is removed from your security stack, <De-Digivolve 1> 1 of your opponent's Digimon. [On Deletion] <Recovery +1 (Deck)>. (Place the top card of your deck on top of your security stack.) Then, 1 of your opponent's Digimon gets -4000 DP for the turn for each card in your security stack.
Lv.6 ShadowSeraphimon EX4-050 R Mega | Virus | Seraph {{DigivolveFromNames|1|Seraphimon}} [Opponent's Turn] When a card is removed from your security stack, <De-Digivolve 1> 1 of your opponent's Digimon. [On Deletion] <Recovery +1 (Deck)>. Then, 1 of your opponent's Digimon gets -4000 DP for the turn for each card in your security stack.
Lv.6 ブラックセラフィモン EX4-050 R 究極体 | ウィルス種 | 熾天使型 【相手のターン】自分のセキュリティが減ったとき、相手のデジモン1体を≪退化1≫。 【消滅時】≪リカバリー+1《デッキ》≫(自分のデッキの上から1枚をセキュリティの上に置く)。その後、このターンの間、自分のセキュリティ1枚ごとに、相手のデジモン1体をDP-4000。