If you cast Nemesis Trap for {B}{B}, you may still target any attacking creature, not just the white attacking creature.

An "attacking creature" is one that has been declared as an attacker this combat, or one that was put onto the battlefield attacking this combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game, or the planeswalker it was attacking has left combat.

If you cast Nemesis Trap during your opponent's declare attackers step, you'll put the token onto the battlefield before the declare blockers step begins and you can block with it this combat. If you cast Nemesis Trap later in the combat phase a, you won't be able to block with the token.

The token you put onto the battlefield copies exactly what was printed on the exiled creature and nothing more (unless it was copying something else or it was a token; see below). It doesn't copy whether the exiled creature was tapped or untapped, whether it had any counters on it or Auras attached to it, or any non-copy effects that changed its power, toughness, types, color, or so on. For example, if Nemesis Trap targets an animated Celestial Colonnade, the token you put onto the battlefield will be a Celestial Colonnade that's just a land. (Because of this, Nemesis Trap has received minor errata to specify that you put a token, not a "creature token," onto the battlefield.)

If the exiled creature was copying something else (for example, if it was a Jwari Shapeshifter), then your token enters as a copy of whatever the exiled creature was copying.

If the exiled creature was a token, your token copies the original characteristics of that token as stated by the effect that put it onto the battlefield.

If the exiled creature has {X} in its mana cost (such as Protean Hydra), X is considered to be zero.

Any "enters" abilities of the exiled creature will trigger when the token is put onto the battlefield. Any "as [this creature] enters" or "[this creature] enters with" abilities of the chosen creature will also work.

If the targeted creature is an illegal target by the time Nemesis Trap resolves, the entire spell doesn't resolve. You won't get a token.