Rulings
Q1: Do I have to choose an opponent's Digimon for this card's End of Opponent's Turn effect?
No, you don't have to choose an opponent's Digimon.
Q2: If I choose an opponent's Digimon that can't attack for this card's End of Opponent's Turn effect, does my opponent attack with that Digimon?
No, they can't attack using a Digimon that can't attack.
Q3: 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 of Opponent's 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 of Opponent's Turn effect when there is already an attacking Digimon, your opponent can't attack with the chosen Digimon.
Q4: Can I choose a Digimon that isn't affected by effects for this card's End of Opponent's 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 of Opponent's Turn effect affects a player, therefore your opponent must attack with the chosen Digimon if it can attack.
Data Sources
Lv.5 Knightmon BT18-069 U Ultimate | Data | Warrior <Blocker> [End of Opponent's Turn] [Once Per Turn] You may choose 1 of your opponent's Digimon. Your opponent attacks with the chosen Digimon. --- Inherited: [All Turns] All your Digimon with [Knightmon] in their texts get +2000 DP.
Lv.5 Knightmon BT18-069 U Ultimate | Data | Warrior <Blocker> [End of Opponent's Turn] [Once Per Turn] You may choose 1 of your opponent's Digimon. Your opponent attacks with the chosen Digimon. --- Inherited: [All Turns] All your Digimon with [Knightmon] in their texts get +2000 DP.
Lv.5 Knightmon BT18-069 U Ultimate | Data | Warrior <Blocker> [End of Opponent's Turn] [Once Per Turn] You may choose 1 of your opponent's Digimon. Your opponent must attacks with the chosen Digimon. --- Inherited: [All Turns] All your Digimon with [Knightmon] in their text get +2000 DP.
Lv.5 ナイトモン BT18-069 U 完全体 | データ種 | 戦士型 ≪ブロッカー≫ 【相手のターン終了時】[ターンに1回]相手のデジモン1体を選べる。相手は、選んだデジモンをアタックさせる。 --- Inherited: 【お互いのターン】「ナイトモン」の記述がある自分のデジモン全てをDP+2000。