I would probably go with bluefin. KDE is great, I myself use aurora on one of my devices, but it can also be kinda fiddley with all of it’s options.
The user has never even used a PC and therefore won’t profit from the familiarity that KDE’s default desktop layout provides. Gnome on the other hand offers a more simplified experience with few options and big icons. All of that might be an asset here. You can use menulibre to hide menu entries from the menu and use the official documentation to remove command line access: https://help.gnome.org/admin/system-admin-guide/stable/lockdown-single-app-mode.html.en
Plus it’s still atomic which I actually think is helpful here. For once all the important system stuff is read only. Secondly if one manages to screw something up you can just rebase.
There is BigBlueButton. It’s more focused in educational usecases (online classes and the like) but it works just fine for everything else. You need to host it yourself, but there are hosted instances out there. I for example use senfcall.
But I think we are talking about different things here. What Chanuk was talking about (I think) is a ms-teams or slack alternative, not a zoom or oracle WebEx alternative. Basically Discord but for business. Sidenote: there is a open source Discord clone called revolt