If you discard Night Assailant, will Night Assailant's own effect allow him to retrieve himself, thus creating unlimited food for Snipe Hunter?

And can you chain D.D. Crow to Sangan/Mystic Tomato/Ect's effect to remove them from play, and negate their searching?

1. No, you can't retrieve the same Night Assailant.

2. Unless it's outside the Damage Step you can chain D.D. Crow to Sangan when sent by other means except by battle (it wouldn't work with Tomato or the likes ever since their effect resolve in the Damage Step), but it would NOT negate it's effect, since it would already be in the grave.