Well, if you want to weaken the shells, you can dissolve out some of the calcium carbonate by soaking in vinegar for a few weeks. But you're right that shells are quite stable in most environments (believe it or not, seawater is one of the harshest environments on the planet, not counting volcanoes, geysers, etc.), so you may have a lot of shells hanging about for a very long time.

Honestly, I think that unless you can find an easy way of pulverizing them (steamrolling, sneaking into an old-fashioned fulling mill, throwing under a train...), it's probably not worth trying to get them to decompose.

I put old 7-9" abalone shells in my bin whole and the worms love them. I think the worms work on the outside of the shell. I put some food in the shell and bury into the top of the bed and cover. A few months later there is a very happy worm family living in the shell. I know the shell will outlast me. I never thought about vinegar. I will try that. Pete

I removed an abolone shell from my bin today and examined it closely. The worms are definitely eating the shell. Soon the shells will be below the legal size of 6 3/4".. I think any whole shells could be added for a source of calcium
Pete