Rulings
Q1: Does this card's digivolution requirement where it can digivolve from a Tamer mean that the Tamer that will become a digivolution card is treated as a Digimon when it digivolves?
No, it digivolves from the Tamer as-is.
Q2: When a Tamer would digivolve using this card's digivolution requirement, is it considered a Digimon that would digivolve?
No, it isn't considered a Digimon that would digivolve. When a Tamer would digivolve using this card's digivolution requirement, "when a Digimon would digivolve" effects don't trigger.
Q3: When a Tamer digivolves using this card's digivolution requirement, is it considered a Digimon that digivolves?
No, it isn't considered a Digimon that digivolves. When a Tamer digivolves using this card's digivolution requirement, "when a Digimon digivolves" effects don't trigger.
Data Sources
Lv.4 Mercurymon BT18-064 C Hybrid | Variable | Mutant [Digivolve] [Sephirothmon]: Cost 0 [On Play] [When Digivolving] Until the end of your opponent's turn, this Digimon can't be returned to the hand or deck by their effects. --- Inherited: [Opponent's Turn] This Digimon gets +2000 DP.
Lv.4 Mercurymon BT18-064 C Hybrid | Variable | Mutant [Digivolve] [Sephirothmon]: Cost 0 [On Play] [When Digivolving] Until the end of your opponent's turn, this Digimon can't be returned to the hand or deck by their effects. --- Inherited: [Opponent's Turn] This Digimon gets +2000 DP.
Lv.4 Mercurymon BT18-064 C Hybrid | Variable | Mutant [Digivolve] [Sephirothmon]: Cost 0 [On Play] [When Digivolving] Until the end of your opponent's turn, this Digimon can't be returned to the hand or deck by your opponent's effects. --- Inherited: [Opponent's Turn] This Digimon gets +2000 DP.
Lv.4 メルキューレモン BT18-064 C ハイブリッド体 | ヴァリアブル種 | 突然変異型 「セフィロトモン」:コスト0 【登場時】【進化時】相手のターン終了まで、このデジモンは相手の効果で手札/デッキに戻らない。 --- Inherited: 【相手のターン】このデジモンをDP+2000。