How does Spine of Ish Sah interact with It That Betrays?
This is a situation for Active Player / Non-Active Player (APNAP) ordering:


  405.3. If an effect puts two or more objects on the stack at the same time, those controlled by the active player are put on lowest, followed by each other player’s objects in APNAP order (see rule 101.4). If a player controls more than one of these objects, that player chooses their relative order on the stack.


The attacking player is the active player, and the defending player is the non-active player. So:


The defending player sacrifices two permanents (including the Spine of Ish Sah) to the Annihilator 2 trigger.
Two instances of It That Betrays' triggered ability go on the stack first (one for the Spine, one for whatever other permanent was sacrificed), in an order chosen by the active player.
Spine of Ish Sah's triggered ability goes on the stack.
Assuming nothing else happens, Spine of Ish Sah's triggered ability resolves first, returning it to its owner's hand.
It That Betrays' triggered abilities resolve, but Spine of Ish Sah isn't in the graveyard anymore, so it won't be put onto the battlefield. The other trigger for the other sacrificed permanent can still put it onto the battlefield, though.


This does mean that if the Spine were somehow sacrificed during player B's turn, the order of the triggers will be reversed, and it'll end up on the battlefield under player A's control.