if i pop the Knuckles and don’t end up completing the boss battle, can i do a different raid without it being used?
Yes. Here’s an example with King’s Fall. Let’s say you’re on the Oryx CP and you use a Knuckles of Eao, however you don’t/are unable to defeat Oryx and now it’s the new weekly Raid, Wrath of the Machine, when you take on Vosik on his second battle and defeat Vosik, the Kunckles of Eao will then be used on Vosik, prior to Oryx. Hope this helps!