Hi and welcome.
Nice illustration. 
For your scenario 2 I am using Personal Backup (only Windows) for years now and I am totally happy with this backup solution (don’t forget to donate if you will be happy with it as well). But there’s one thing this tool cannot do, and this is directly connecting to backblaze or Google drive (only local and network or FTP).
And even if it could, that would mean to download your vault from gdrive and upload it to backblaze (in your Szenario incremental). Nevertheless I cannot see the benefit of doing this when you have all your encrypted gdrive files already local available (as stream). So why not do the incremental backup of the encrypted files from your local gdrive to backblaze?
And I like to point out that the vault structure (means the structure of encrypted files) is crucial for decryption your files. If you mess it up when restoring from an incremental backup, you will most likely loose files.
I recommend to have a mirror of your encrypted vault on backblaze instead of an incremental backup. You should use your unencrypted Lokal/externalHDD backups to keep file versions. (Just my 2 cents).
Also, please not that it is not possible to just „select some folders for backup“ in an encrypted state, as you do not know which encrypted files and folders belong to which unencrypted file. Please always backup your complete vault if you want to make a backup of your encrypted data.
Hopefully I didnt get anything wrong in your description.