The clam shells are a good substitute for oyster shell. I believe you call that soluble grit in the UK. So yes, it can be used as soluble grit when they start to lay.
Insoluble grit is used in the gizzard to grind stuff they eat since they don't have teeth to do that for them. The clam shells and oyster shells do not work for that. The digestive juices will dissolve them. Put a clam shell in vinegar, you will see what I mean. They need rocks for that. The insoluble grit you can buy is granite, wastes from a granite quarry that has been screened for size and washed. I don't know what you native rocks are, they can probably use those. Just give them access to the ground and the will find their own.
Sea gulls and other salt water birds have a special gland to filter out excess salt. Chickens don't have that gland, excess salt can harm them. I don't know enough about your specific seaweed to know if it is suitable for them or not but I'd rinse the salt off of it before I offered it to them. I probably would offer it to them to see what they do. Some seaweed is highly nutritious.