Q1: If this Digimon attacks when this card's On PlayWhen Attacking effect prevents it from being affected by the effects of my opponent's Digimon, can it be blocked by my opponent's Digimon?

Yes, it can be blocked. Blocker is an effect that allows for the Digimon to block, so it isn't affected by ""isn't affected by the effects of your opponent's Digimon"" effects.

Q2: 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.

Q3: 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.

Q4: 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.

Q5: 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.

Q6: Does this card's On Play When Digivolving effect prevent Security effects on my opponent's Security Digimon from affecting this Digimon?

Yes, it isn't affected. Security effects on Security Digimon are treated as Digimon effects, so the "this Digimon isn't affected by the effects of your opponent's Digimon" effect prevents such effects from affecting this Digimon.