Death save specifically refers to banging an already drained (right outlane) ball off the left side of the apron to put it back into play. Doesn't matter if the game slides or not, the key is the ball has already drained below the flippers.

Bangback is a deathsave, just from the left side, so it's harder to pull off.

Slide save is a more generic term used anytime the game actually slides along the floor. As long as the ball is still in play they're legal, but usually result in tilts unless the bob is set liberal or there's debounce code.

The posts themselves are in play. Ball position in the lane doesn't really matter.. games like CV have the posts low in the lane, and Centaur gives you a post to try knocking it back into play. If you can influence the ball back into play with the posts without tilting, it's legal. The posts are intended for that purpose. The apron is NOT intended for that purpose.