If you exile your entire library without exiling a creature card, you put the library back in a random order and continue. You won’t return the exiled attacking creature. Oops.

You choose which player or planeswalker the new creature is attacking. It doesn’t have to be attacking the same player or planeswalker that Fireflux Squad or the exiled creature was attacking.

Although the new 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.

Any effects that say that the new creature can’t attack (such as that of Propaganda, or if you get a creature with defender) affect only the declaration of attackers. They won’t stop the creature you find from entering the battlefield attacking.