This is due to the improved way that LinuxServer handles users in their setups, where you run as less privileged users. Because of that, you may want to create a specific backup user/group and use that from within the image.
The Duplicati image does not have that option because it runs as root, and thus has full access to the mounted volume, regardless of the actual owner.
No, not in the current version, but I think you are right this should be possible.
I have written up a PR that adds UID/GID support to the Duplicati Docker images.