Usually, I’m linking the Nextcloud folder at the categories pinned introduction topic. But I also like to share folder content within any other post to offer instant access for course material / project files and more.
I guess not. Why is this relevant? It might be possible some NC folder sharings could occur multiple times within one Discourse instance.
Read-only would be perfect. I need the information about file/folder names, file types, sizes, the option to expend / collapse sub folders and a direct download link.
In case the shared folder is password protected, the PW and direct link to NC should be noted somewhere beside the listing view.
It’s a complicated and fair question. I’m not very experienced in (web-based) software development and the cash flow is always an issue 
I have no commercial use-case. I’d like to close the gap between course document management and an excellent community experience. Each course has it’s own category Moodle and OpenOLAT have a bad user experience.
I would say, yes.
Having a seamless user experience in discussion and document management is the goal for integrative / barrier-free “groupware”. I would compare it with the back linking (feedback loop) of Discourse’s github commits at the comments section on github.com. Every time the users need to open the link separately on anther tab / window, he/she will be interrupted and gets confused from different platforms and usabilities.