Multiple creatures dying will cause Bereaved Survivor to transform only once.

You choose which player or planeswalker the returned creature is attacking. It doesn't have to be attacking the same player or planeswalker Dauntless Avenger is attacking.

Effects that say that the returned creature can't attack (if the creature has defender, for example) affect only the declaration of attackers. They won't stop the returned creature from entering the battlefield attacking.

Although the returned creature is an attacking creature, it was never declared as an attacking creature. This means that abilities that trigger whenever a creature attacks won't trigger when it enters the battlefield attacking.