The reason you're seeing horseshoe crab shells on the flats is because they must "molt" or shed its shell in order to grow larger. Horseshoe crabs molt several times during its first few years alive. After its third or fourth year it sheds its skin annually.
I have not seen redfish eat horseshoe crabs in person, nor do I know anybody who has. I never heard of anybody using them as live or cut bait. They do use horseshoe crabs blood for medical reasons. Some laws protect horseshoe crabs from being caught alive and sold because of that.
I have been around redfish in the wild and inside the FWC hatchery and I would bet that if a horseshoe crab is dead, a redfish tailing or rooting around a dead one, would flip it over and eat whatever is inside the horseshoe crab it could. That is what redfish do. I have caught redfish on every type of crustacean, live bait and cut bait. Some frozen baits over 5 years old. The only way they might not eat a horseshoe crab is alive and whole because of the long hard telson (the horseshoe crabs hard tail). It would be difficult even to crush and get into their inferior mouth. Harvesting of horseshoe crabs is prohibited.
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