Sacrifice

Q. Can I kill a unit equipped with a [Guardian Angel] to pay the additional cost with my unit ending being saved? Or with a Zhonya ? Or with Sett's legend effect?

A. Yes. As opposed to [Deathgrip] which reads "If you do, ...", the additional cost of killing the unit for [Sacrifice] would be considered paid.

Q: I use <<Sacrifice>> to kill a Mighty unit as an additional cost. Can I trigger <<Immortal Phoenix>> from my Trash?

Rules for Context - 135.2.b.2., 135.2.b.3., 428.1.a.1., 428.5.e.

A: Yes.

Q. You play Sharkling accelerated, and move it into a battlefield against a Sprite token, initiating combat. Are you able to play Sacrifice and kill Sharkling AFTER it kills the Sprite token?

No.

813.1.C.1 Reaction, additionally, is functionnaly short for the following:
On Cards: "This can be played during Closed States on any player's turn."

309.1 If a Chain exists, the turn is in a Closed State.

In other words, you must have a thing to react to in order to play a reaction. In the given case, there is no Chain to react to.