3 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

Can I choose an opponent's Digimon with a "this Digimon can't attack" effect as the target for "<Start Your Main> Attack with this Digimon" when I activate this card's <Main Timing> effect?
Yes, you can. However, in such cases, the attack isn't performed due to the "this Digimon can't attack" effect.
Ruled Mar 28, 2024
What happens if I use this card twice and 2 of my opponent's Digimon gain the "<Start Your Main> Attack with this Digimon" effect?
At the start of your opponent's main phase, the effects gained from this card's effect are triggered at the same time, and your opponent determines which one activates first. After effects such as <When Attacking> have activated, and before the counter timing occurs, it is time to activate the effect of the 2nd Digimon that was pending activation. However, since the 1st Digimon is already attacking, the effect ends without an attack made. As a result, only the Digimon whose effect was activated first can attack.
Ruled Mar 28, 2024
Can I choose an opponent's Digimon with an "<Opp Turn> This Digimon isn't affected by your opponent’s effects" effect as the target for "<Start Your Main> Attack with this Digimon" when I activate this card's <Main Timing> effect?
Yes, you can. In such cases, as soon as it becomes your opponent's turn, the target opposing Digimon loses the "this Digimon isn't affected by your opponent’s effects" effect, and the attack occurs at the start of the main phase by this card's effect.
Ruled Mar 28, 2024
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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.