I think you could safely add them to your trenches in the same way that I add pulverized egg shells to my planting holes - not to help the plants this year, but to feed the worms and help my garden in the future.
The only caveat with the crab shells would be that they smell might attract wild animals or dogs to your garden.
A few years ago, my dog kept going next door, sniffing and digging in my neigbours shrubs! Several months later I realized what had enticed her over there when daffodils sprang up and I guessed that he had used bonemeal at planting time!!!
Linda
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