Joe,
Although I have processed thousands of cow horns into powderhorns and other things, I have never had the dubious pleasure of removing the core. I think a long flexible, narrow knife would be a big help. As would a 4 x 1/2" lag bolt to screw into the core. A 1/4" bolt might work easier. And it could be longer as well.
I am thinking someone posted a video here a few months ago showing how it was done.
I would guess boiling it maybe 10 minutes should be adequate. As you say, you have had these horns some time, so the membrane could even be non-existent by now.
Hope these few thoughts help maybe even a little bit.
John (Bigsmoke)
P. S. Would love to come visit you, but would not walk across the street to help you with this project. Sorry, but I have managed to avoid boiling horns to remove cores for over 35 years. Don't really want to start now.
It was bad enough rough and fine sanding them all.