I don't recognize that S3 storage class, but I don't think that should matter. Are you doing a file folder back up or an image backup. When you created the backup in version 7, assuming that's where you created it, did you just start the wizard normally or did you use the drop-down to select the legacy file folder back up format? The reason that's important is because the new file format archives a bunch of files together into a single file and then uploads those larger files to the cloud so the backups and restores occur more quickly. It's not a single file for the entire backup, but just a larger file overall compared to backing up thousands or hundreds of thousands of very small files individually. Depending on what's getting backed up using the new file format it's possible that all the changes fit into a single archive file and that's why you only see a single file moving up to the cloud.
The multi-threading is controlled in the options dialog I believe under advanced where you can see the number of threads. That will determine the maximum number of file streams that are processed in parallel.
As far as your version you say desktop, but I need to know if it's the free version or you paid for the pro version. The free version only supports a single thread to the cloud. Whereas the pro version supports multiple streams.