Have you tried percussive maintenance? That deck fitting has to come off, one way or another. Mine was stuck as well, I twisted one of those multi tools
like a pretzyl. Big screwdriver, rubber mallet, several counterclockwise taps ....viola! Rewarded myself with another big screwdriver, over ice.
Sounds like yours is more stuck than mine was, but the suggestion is hammering on it tangentially to break the oxidization.