A solution is to restore and use Manjaro's boot list.
At that other linux grub menu, go into its grub prompt (grub>) by pressing 'c'
At its grub prompt,
grub> search.file /boot/intel-ucode.img root grub> configfile /boot/grub/grub.cfg
Manjaro grub menu will then appear, enter at the first (top) entry and we will boot into Manjaro.
When booted into Manjaro, at terminal,
sudo grub-install /dev/sda sudo update-grub
After restart the Manjaro boot list will be accessible directly.