Unable to remove path prefix on language when changed to from default language

Steps to reproduce:

- Install Drupal in English
- Enable language module.
- Add new language (for example Norwegian, in my case)
- Make Norwegian the default language.
- Go to detection and selection screen.
- Try to remove path prefix for Norwegian.
- Error message appears. It says the path prefix can only be left blank for the "default fallback language".
- I go to look at the "default fallback language". It says it is Norwegian.
- If I save the form (just save it, without doing any changes) I can now go back and remove the path prefix (gif attached to illustrate)

As I see it, there is a couple of different issues here:
1. Were it not for the fact that I have used and developed for Drupal for quite a few years, I would have probably thought that the error message about "default fallback language" was about the "default language" (found in the form over at admin/config/regional/language). The form that relates to this error is found under the wording "Selected language - Language based on a selected language.".
2. Could we just have a link in the error message to the correct form? (Created #2486293: Create link to "selected detection fallback language" from error message. for this).
3. Of course the most obvious problem. I go to the correct form, and to me it seems the default fallback language _is_ indeed Norwegian (and it is), but the path removal verification does not pick it up. (Created #2486301: Allow removal of path prefix for language if it set to default language of site, but no config key for "selected_langcode" exists. for this).
4. If you install a site in (for example) Norwegian, it is automatically assumed and made "path-prefix-free" if I add another language. Can we do something similar in either the changing of default language or installation of the language module? (Not sure what is best or most user friendly here).

Should this maybe be different issues?

Would love to hear some thoughts on this, and am happy to provide a patch for whatever direction we end up going :)