Spells can only be cast for their sneak costs any time you could play an instant during the declare blockers step on your turn (after your opponent has decided whether to block).

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

If a creature spell's sneak cost was paid, the creature it becomes enters tapped and attacking the same player, planeswalker, or battle as the creature that was returned to its owner's hand to pay its sneak cost. This is a rule specific to sneak; in other cases, when a creature is put onto the battlefield attacking, that creature's controller chooses which player, planeswalker, or battle it's attacking.

Auras attached to the exiled creature will be put into their owners' graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist. If the card returns to the battlefield, it will be a new object with no connection to the card that was exiled.

If Turncoat Kunoichi's sneak cost wasn't paid and it leaves the battlefield before its triggered ability resolves, the target creature won't be exiled.

Although a creature put onto the battlefield this way 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.