Some people do put paper in the compost pile (tissues, napkins, etc.) Certainly they will decompose. The shells will stay whole for an eternity. Every now and then I dig up a clam shell, in my yard- undamaged. It has probably been in the dirt for 50 plus years. You can add the shells- I would just crush them first, if you want them to disappear. Having them whole in your compost would probably still add some minerals to the mixture. (You could just pick them out, when you are using the compost in the garden, if they bother you...)
Yes, that's a good use of something that normally would be trashed. One of the posters here put all their shredded paper into buckets of water to start breaking down before using as mulch in their garden.
G'day,we put everything that "rots" into the compost even what comes out of the vacuum cleaner,If you can get it I once heard that human hair makes great compost if you live near to a hairdresser.................T.O.R..................