Grab a can of "plus gas", spray liberally, leave to soak, spray again, now try tightening the COV (with whatever cheater bar/massive spanner/etc. you've got. If it moves, you're away; you'll find that although stiff, it should screw right out.
I know, I know, there are some around here who think these penetrating fluids are snake oil. However, as I say to anyone who asks, I dismantled, bolt by bolt, a 1977 Jaguar XJS which had been sitting outside, not moved, for nearly 20 years, and I only snapped one bolt. Even the ones which looked like they were more iron oxide than iron, yielded to the plus gas. I swear by the stuff (and very rarely at it).