What sort of url is that, you need to pass an absolute URL like so:
http://yourdomain.com/xyz or
http://localhost/kirby
You cannot use a relative link.
No, you can’t. But as I said above, you can create a custom select field that fetches the options from the tags field and passes them as options.
Edit: See this post for an example: Fetch query from parent page - #2 by texnixe