node, node package manager and yarn is not a part of and not included with a pristine Manjaro system.
No
Not that I know of. But I don’t know everything - so it may be possible …
In the topic [root tip] [How To] Do a manual Manjaro installation it is described how to install the packages from a live ISO - recreating the ISO manually - in theory - this could be done on an existing system as well.
I don’t know if it will help in your case - if it doesn’t help - it has done no harm
cat /rootfs-pkgs.txt | awk '{print $1;}' > ~/iso-pkglist.txt
cat /desktopfs-pkgs.txt | awk '{print $1;}' >> ~/iso-pkglist.txt
pacman -Syu --noconfirm --needed - < ~/iso-pkglist.txt