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It's certainly possible to design such a system with external firmware and still allow for secure updates.

The enclave would store (in secured storage) a hash of the last used firmware. Hardware would have a hash update capability, but this destroys all other stored information (i.e., keys) if used when the enclave is not currently in an unlocked state.

On boot, hardware verifies firmware signature as usual but also compares the firmware hash (already calculated for the signature check) to the stored value. If there is a mismatch, update the stored hash. Since the enclave is currently locked, the hardware clears the keys.

Since it's in hardware, you're correct that it would have to be 100% right, but that's quite feasible for a simple update mechanism (indeed, the most complicated bits are reused pieces from the signature check which already has this requirement).