If you search on google and the result seems deleted modify the URL like archived.forum.manjaro.org so add the archived insted. This is due to the prvious forum having had a major disaster and got placed into frozen archive.
For grub - if you only need the menu one time - keep hittin “CAPS” and/or “ESC” key during start - it should bring the menu. I spam them both to get into grub if needed.
if for a permanent change - its something with grub env variables - thats to be found on archived.forum.manjaro.org