When you delete data from the Backup Storage tab, you delete if from cloud storage. We don't store any of your data. It all goes directly to you cloud storage account
You signed up through a B2 website, so you don't see other options. You can create another account directly from msp360.com to see other storage providers
FFI is not supported for local backup. With the new backup format, you must have at least two full backups with increments. For six months of retention with two full backups, you can set monthly full to repeat every six months.
Consistency check compares the list of files to identify if some objects are missing in backup storage or if there is a mismatch in object sizes or modification dates.
The workaround you have set up is the only way to use it as a storage destination. You can also try to install to deploy Minio to your NAS, but not sure if it will have enough resources to accommodate it since it's an old device.