Fallen Shinobi

Fallen Shinobi's triggered ability doesn't change when you can play the exiled cards. For example, if a sorcery card is exiled, you can cast it only during your main phase when the stack is empty. If a land card is exiled, you can play it only during your main phase and only if you have an available land play remaining.

Although the Ninja is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).

As you activate a ninjutsu ability, you reveal the Ninja card in your hand and return the attacking creature. The Ninja isn't put onto the battlefield until the ability resolves. If it leaves your hand before then, it won't enter the battlefield at all.