Collapsing Trap (Blue)

“When this defends” is an event-based trigger. Whether the attack has go again is checked at the time this resolves and becomes a defending card (i.e. defends the attack). If the trigger condition is met only after this resolves, then this does not trigger.

If there are fewer cards in the hero’s deck than how many they would draw, they draw all of those cards.

If an effect triggers whenever a card is discarded (specifically one and not “one or more”), it triggers for each card discarded this way (e.g. Rhinar).