Should everything be severely damaged, Duplicati ships with RecoveryTool which is designed to enable restores, even on seriously damaged backup storage.
If you prefer, there is also a Python script that can do recovery:
That does not sound correct. Duplicati will generally need one folder per backup task. You can add/remove sources to the backup, but each backup configuration needs a single remote folder.
Decryption is the easy part. You can use any tool that implements the AESCrypt specification, and Duplicati ships with one.
If you want to restore data, you need to understand how Duplicati works internally. This document essentially describes what the recovery tool does: