Q1: Can this card's End of Opponent's Turn effect not select an opponent's Digimon?

Yes, it is possible not to choose.

Q2: If this card's End of Opponent's Turn effect selects a Digimon that cannot attack the opponent, does the opponent still attack that Digimon?

No, you cannot make a Digimon attack that cannot attack.

Q3: At the end of opponent's turn, the situation is that opponent's Digimon is attacking with an effect. If this card's End of Opponent's Turn effect selects an opponent's Digimon, does the opponent let that Digimon attack?

No. Your opponent cannot make that Digimon attack. While attacking, you cannot declare a new attack. In this situation, if there is a Digimon attacking and you choose one of your opponent's Digimon with this card's End of Opponent's Turn effect, your opponent cannot make that Digimon attack.

Q4: Can I select a Digimon that is not affected by the card's End of Opponent's Turn effect and have it attack?

Yes, you can choose a Digimon that is not affected by the effect and have it attack. Digimon that are not affected by the effect are not affected by the effect, but can be chosen. Also, the End of Opponent's Turn effect of this card End of Opponent's Turn, "Opponent can make the chosen Digimon attack" affects the player, so the opponent must make the chosen Digimon attack if it is able to attack.