Hi,
I already found the first link you posted. I didn’t tried but it seem to integrate only with the “external site” feature of nextcloud, but I don’t see how it does unified authentication. Probably it doesn’t. It also add nextcloud contacts into roundcube via a carddav plugin.
The second link is only about integrating nextcloud contacts into roundcube via a carddav plugin. It doesn’t look like it talks about unified authentication.
The last link seem like it’s an abandoned not-working project. Last commit is about a year ago and the readme say: “will not automatically login at this point. It does logout of your nextcloud seesion when you logout of Roundcube. I just started this thing”. Unfortunately, this is exactly what I would need.