Since your inserts are already spinning I would guess that whatever they were screwed into has failed. What is your plan for installing the new ones? Did you order them oversized so they will bite into whatever was holding the old ones or are you planning to fill the holes and re-drill them to the right size for the new inserts.
I too have not worried too much about the galvanic corrosion
until recently though I did have concerns about it. About 2 months ago I had a fitting in an air conditioner T develop a pin hole leak. Before I heard the bilge pump
go off it managed to spray salt water
mist over my entire starboard engine
room. Even though I washed down everything with fresh water
I now have rust all over the place. So much for my 18 year old engine
without a spot of rust. I'm just hoping that no salt water
got down the threads of the mounting bolts.