I don’t think you can start manually Manjaro Pacnew Checker (anymore?).
After an update, if the post update hook script finds .pacnew files, it creates a temporary file, and if this temporary file exists, then it auto starts Manjaro Pacnew Checker (because there is a service periodically checking if this temp file exists).
You can, for a test, copy the file, let say /etc/makepkg.conf, to /etc/makepkg.conf.pacnew, then trigger an installation, for example sudo pacman -Syu nano, it will detect .pacnew file, and after a few seconds it will auto start Manjaro Pacnew Checker.
PS: I guess for further issues with Pacman configuration about repositories, a new thread could be created (or all the posts could be moved to new thread). This is kinda off topic.