If the "intervening if" clause of Suppressor Skyguard's last ability is no longer met when the ability tries to resolve, it won't resolve and none of its effects will happen. For example, if all of the attacking player's other opponents leave the game before Suppressor Skyguard's last ability resolves, the condition will no longer be true when the ability tries to resolve, the ability won't resolve, and damage won't be prevented.

Suppressor Skyguard's ability will trigger if any of the attacking player's opponents besides you aren't being attacked. It doesn't matter if that player is also attacking any planeswalkers those opponents control or battles they protect.