You choose the player, planeswalker, or battle the returned creature is attacking. It doesn't need to be the same player, planeswalker, or battle that The Neutrinos or any other attacking creatures are attacking.

If a token is exiled this way, it will cease to exist and won't return to the battlefield.

Once a creature exiled this way returns, it's considered a new object with no relation to the object that it was. Auras attached to the exiled creature will be put into their owners' graveyards. Equipment attached to the exiled permanent will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist.

Although the creature you put onto the battlefield is attacking, it was never declared as an attacking creature. Abilities that trigger whenever a creature attacks won't trigger when that creature enters attacking. (Any abilities that triggered before The Neutrinos exiled the creature are unaffected, and the source of those abilities is that creature as it last existed on the battlefield, not the creature that's attacking now.)