When all else fails...
You'll find directions for freeing a stuck macerator in the "manual" for it:
http://www.ittflowcontrol.com//files/18590_series_sealed_macerator.pdf But I wouldn't bother to try too hard to revive it, 'cuz after sitting unused for that many years, it's toast Sitting unused is far more destructive to equipment, especially electric motors, than constant hard use. Since you're not using it, I'd simply remove it and the plumbing to the thru-hull.
Antifreeze is sticky...which is most likely what's "welded" the impeller to the housing. The whole system should be thoroughly flushed out with plenty of clean water in preparation to winterize and the thru-hull left open and/or hose disconnected so that any water can drain instead of leaving antifreeze in it for the winter.
It's in one of those very hard to reach areas of the boat so I am dreading a replacement...
That tells me you aren't checking the hose connections or seacock for deterioration or signs of impending failure/need to rebed or lube. So whatcha gonna do if/when you start taking on water in that "hard to reach area"...call the CG to rescue you and then just watch her sink? :naughty: You wouldn't be the first to do that 'cuz maintaining a boat requires a lot more effort than many people expect...so they don't do it. And then don't know what to do when they find out the hard way that they should have.
So if you're not using the macerator, don't just leave it in there to fall apart and sink your boat...bite the bullet and pull it AND all it's related plumbing out. And check the thru-hull and seacock while you've shoe-horned yourself into position to do it.