"The question I would have is, if data synthesis, legal risk-based de-identification, differential privacy, and cryptographic tokenization protocols were insuff

"The question I would have is, if data synthesis, legal risk-based de-identification, differential privacy, and cryptographic tokenization protocols were insufficient, what technical improvement in actual accountability does FHE offer to data subjects, and given the size of the data sets this facilitates, what are the consequences of its failure modes?" It doesn't. Because its not aiming at solving these problems. Encryption in-use is aiming to solve trusting hardware (and maybe code) you don't own. Privacy is a different (IMO more complex) problem.