Hi @kazoo
I’ll start with what I hope is the easier one. 
Your #2 is because Duplicati doesn’t support SMB URL syntax. Actually, it doesn’t support SMB by itself, but might someday. On Windows, SMB support is by the filesystem, therefore it even works at a Command Prompt. A Linux shell can’t do that. Your File Manager (is it Nautilus?) appears to do the necessary mounts on your behalf. Duplicati needs mount help. What options did you set?
Running the mount command with no options (or doing cat /proc/mounts) will get a list you can pick from.
Typically a file manager would run as an ordinary user. Is Duplicati running as root or you on the failing test?
For issues that look like access, I may ask what Duplicati is running as, then ask for a manual test from shell.
For example, can you, as whatever Duplicati is, make directories and files (new ones to be safe) in the area?
If that works, then the next test would be existing files, but test on a nonessential one if you’re uncomfortable.
Thanks!