If you get these errors, please enter the whole line in a terminal and check the output for errors.
No, the documentation should provide all you need.
No need to point to the Nethserver ejabberd. If configured as described in the docs, it should just work.
Maybe. Is the ejabberd service running?
systemctl status ejabberd
Does it work if you connect from another client (to check if it’s an ejabberd or webtop issue) ?
I’d try the other things first but if nothing works, reinstalling an app is always worth a try.