Thank you for such a complete response and for your time.
Meanwhile, you have to keep Duplicati from wanting access to old data even to sanity-check things. –no-backend-verification can do that but it will raise risk. –no-auto-compact should also be used.
Thanks for pointing that out! 
How would you test the backup if you can’t restore sample source files or even verify sample backend?
With this great script running in the NAS: Complete, incremental or differential - #12 by kenkendk
I don’t understand all the talk about hashes and hash lists. There are many hashes. Which do you mean?
The hashes of the backed data (I think dblock files store the backed data, right?), which allow verifying that the information is the same and that its content has not changed without storing the data itself. Therefore, from the verification point of view, having the hashes is equivalent to having the data (but with much smaller storage of information, and assuming both that the data has been correctly stored and that it matches the hashes).
Similarly I don’t believe (but you could test if you like) that Duplicati needs to have files in the destination to upload changes.
If I run duplicati without --no-backend-verification it raises an error of missing files, I think because it verifies the content of the destination before uploading further changes.
I’ll tell you about the POC when --no-backend-verification is enabled. After uploading a couple of backup files from the could, I have deleted some information, and I could continue uploading the backup with further changes. Similarly, I was able to restore to a previous version only (as you pointed) if I upload the necessary version files from the NAS to the cloud. I did the same process but deleting all the files from the cloud instead of only some, and I was able to upload and restore backups as well. I didn’t need the --no-compact option so far in Keep all backups mode, but I will enable it anyway.
The NAS is trying to help by reducing cloud storage amounts. How is the cloud helping in this scenario?
Unlike the NAS, the cloud provides high availability with zero maintenance. It also isolates the NAS from the outside world, since it does not need to accept incoming connections as a server (except in case of a disaster), but to download the necessary files from the cloud once a week. Finally, it opens the door to 1-to-many backup schemes. I think these are a set of interesting advantages to consider 