Sorry to hear that none of the restoration options were available for you.
Your Codebook database is encrypted locally (on device) using your Codebook Master Password as key material. When syncing, Codebook encrypts “chunks of changes” (codebook-changeset-[number].db files in your cloud service) with your Sync Key. So to answer your question directly – no. Synced data is encrypted once with the Sync Key.
I believe this post by my colleague helps explain why we made the decision to use a Sync Key to encrypt synced data: Why Does Codebook Require a Sync Key?
Let me know if that answers your questions and if you have any further questions. Thanks!