I think we should also add a parsing check for slot names specified in the GUC synchronize_standby_slots as suggested by Amit in [1]. I made the changes in the above for the same and attached the updated patch.
I agree, validating that list contains valid replication slot names is a good idea.
However, you used ReplicationSlotValidateName() function, which is not a good fit for it, especially when it is called with elevel=ERROR in postmaster.