Q1: Can I use this card's digivolution requirement to digivolve for a cost of 5 even when my Koji Minamoto has more than 5 Hybrid trait cards under it?
Yes, it can digivolve.
Q2: Does this card's All Turns effect trigger when either my Digimon or my opponent's Digimon attacks?
Yes, it triggers.
Lv.6 MagnaGarurumon BT18-042 SR Hybrid | Variable | Cyborg [Digivolve] [Koji Minamoto] w/5 [Hybrid] trait cards under: Cost 5 [When Digivolving] [End of Opponent's Turn] [Once Per Turn] By placing 1 Digimon card from this Digimon's digivolution cards as the bottom security card, delete all of your opponent's Digimon with the same level as the placed card. [All Turns] [Once Per Turn] When Digimon attack, by adding your top security card to the hand, this Digimon unsuspends.
Lv.6 MagnaGarurumon BT18-042 SR Hybrid | Variable | Cyborg [Digivolve] [Koji Minamoto] w/5 [Hybrid] trait cards under: Cost 5 [When Digivolving] [End of Opponent's Turn] [Once Per Turn] By placing 1 Digimon card from this Digimon's digivolution cards as the bottom security card, delete all of your opponent's Digimon with the same level as the placed card. [All Turns] [Once Per Turn] When Digimon attack, by adding your top security card to the hand, this Digimon unsuspends.
Lv.6 MagnaGarurumon BT18-042 SR Hybrid | Variable | Cyborg [Digivolve] [Koji Minamoto] w/5 [Hybrid] trait cards under: Cost 5 [When Digivolving] [End of Opponent's Turn] [Once Per Turn] By placing 1 Digimon card from this Digimon's digivolution cards as your bottom security card, delete all of your opponent's Digimon with the same level as the placed card. [All Turns] [Once Per Turn] When a Digimon attacks, by adding the top card of your security stack to the hand, unsuspend this Digimon.
Lv.6 マグナガルルモン BT18-042 SR ハイブリッド体 | ヴァリアブル種 | サイボーグ型 特徴に「ハイブリッド体」を持つカードが下に5枚ある「源輝二」:コスト5 【進化時】【相手のターン終了時】[ターンに1回]このデジモンの進化元から、デジモンカード1枚をセキュリティの下に置くことで、置いたカードと同じLv.の相手のデジモン全てを消滅させる。 【お互いのターン】[ターンに1回]デジモンがアタックしたとき、自分のセキュリティを上から1枚手札に加えることで、このデジモンをアクティブにする。