4 official rulings from Bandai. Straight answers to how this card actually resolves.

[BT2-079]/[VenomMyotismon]'s effect is "<Opp Turn> When the opponent's Digimon suspends". The opponent's Digimon is rested to attack. Which comes first <When Attacking> or the <Opp Turn> effect of VenomMyotismon to increase memory by 1? Is it the same timing as <When Attacking>?
Since the effect of [BT2-079]/[VenomMyotismon] and <When Attacking> would be resolved at the same time, the opponent's effect will be resolved first, and then VenomMyotismon's effect will be resolved.
Is [BT2-079]/[VenomMyotismon]'s <Opp Turn> effect considered a [Reaction] effect?
No, it is triggered as soon as the opponent declares an attack suspending the Digimon. (See more in Attack Resolution)
My opponent attacks and activates an effect that <De Digivolve> on my Level 7 Digimon that becomes a [BT2-079]/[VenomMyotismon]. Can I use VenomMyotismon's effect?
No, you cannot. The effect of this card was not triggered when the attack was declared (which caused the Digimon to suspend), so it cannot be activated. (See more in Effect Resolution)
My opponent attacks and activates an effect that <De Digivolve> on my [BT2-079]/[VenomMyotismon]. Can I use VenomMyotismon's effect?
No, you cannot. Even though the effect of [BT2-079]/[VenomMyotismon] was triggered, the Digimon has lost the <Opp Turn> effect of VenomMyotismon so it cannot be activated. (See more in Effect Resolution)
Got a line that uses VenomMyotismon? Map it in the builder and share it with a link.
Questions and answers are Bandai's official rulings, collected and translated by the DigimonCardGame Wiki community and reused here under CC BY-SA. Card names and card text are © their respective publishers; Combalo is not affiliated with Bandai.