4 official rulings from Bandai, last ruled Nov 1, 2024. Straight answers to how this card actually resolves.

Do I have to choose an opponent's Digimon for this card's <End Opp Turn> effect?
No, you don't have to choose an opponent's Digimon.
Ruled Nov 1, 2024
If I choose an opponent's Digimon that can't attack for this card's <End Opp Turn> effect, does my opponent attack with that Digimon?
No, they can't attack using a Digimon that can't attack.
Ruled Nov 1, 2024
An opponent's Digimon is attacking using an effect at the end of their turn. If I choose an opponent's Digimon for this card's <End Opp Turn> effect, does my opponent attack with that Digimon?
No, your opponent can't attack using that Digimon. A new attack declaration can't be made during an attack. In such cases, even if you choose an opponent's Digimon for this card's <End Opp Turn> effect when there is already an attacking Digimon, your opponent can't attack with the chosen Digimon.
Ruled Nov 1, 2024
Can I choose a Digimon that isn't affected by effects for this card's <End Opp Turn> effect and have it attack?
Yes, you can choose a Digimon that isn't affected by effects for this and have it attack. Such Digimon aren't affected by effects, but they can be chosen. In addition, "your opponent attacks with the chosen Digimon" in this card's <End Opp Turn> effect affects a player, therefore your opponent must attack with the chosen Digimon if it can attack.
Ruled Nov 1, 2024
Got a line that uses Knightmon? 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.