Verified answer

1) Your friend declares an attack on your face-down Man Eater Bug with one of his Skilled Dark Magicians.

2) You activate Staunch Defender and select Wall of Illusion.

3) A replay occurs. If your friend chooses to attack, then he must attack your Wall of Illusion.

4) Let's say he still chooses to attack. His Skilled Dark Magician will destroy your Wall of Illusion (because its ATK is higher than Wall of Illusion's DEF). After damage calculation, the Skilled Dark Magician that attacked will be returned to his hand due to Wall of Illusion's effect.

5) Since the monster selected by Staunch Defender is no longer on the field, your friend cannot declare any more attacks.

End result will be:

You have one face-down card, Man Eater Bug. Wall of Illusion is in the Graveyard.

Your friend will have two Skilled Dark Magicians on the field (neither can attack), and one in his hand.

If you had Neo-Spacian Grand Mole instead, then the same thing would happen, except Neo-Spacian Grand Mole would return to your hand instead of being destroyed (assuming you choose to activate Mole's effect).

That first Skilled Dark Magician will be forced to attack Wall of Illusion but will destroy it since it's 1900 ATK vs 1850 DEF. That Skilled Dark Magician will be returned to hand as per Wall of Illusion's effect. As Staunch Defender's rulings state, if the selected monster is destroyed or removed from the field, then the opponent's other monsters are unable to declare attacks. So the other two Skilled Dark Magicians will remain on the field and do nothing that combat phase.

If they had lower ATKs, they would each be forced to attack it in turn and all returned to hand.

If it were Neo-Spacian Grand Mole then it'd be up to you what happens.

- Either you allow it to be destroyed by battle. The other two Skilled Dark Magicians won't be able to do anything.

- You use Grand Mole's effect to return itself and the attacker to hand. Again, the other two Skilled Dark Magicians can't do anything.