No, you don't have to choose an opponent's Digimon.

No, they can't attack using a Digimon that can't attack.

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 inherited effect when there is already an attacking Digimon, your opponent can't attack with the chosen Digimon.

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 inherited effect affects a player, therefore your opponent must attack with the chosen Digimon if it can attack.