in short, manjaro's grub can boot ubuntu but ubuntu's grub cannot boot manjaro.
no, should be fine. when you restore grub, it will detect your other os's on the system.
assuming you use UEFI:
-boot into live environment
-connnect networking (wifi/ethernet/etc..)
sudo blkid -o list -c /dev/null
-take note of your drive layout, what partition the /efi/boot is located (usually sda1, but make sure)
-in terminal do the following:
mhwd-chroot -a
-enter pw for live environment "manjaro" and if prompted select which drive/partition manjaro is installed on, it should go something like thise:
[dglt@dglt-2 ~]$ manjaro-chroot -a
[sudo] password for dglt:
==> Mounting (ManjaroLinux) [/dev/sdb2]
--> mount: [/mnt]
--> mount: [/mnt/boot/efi]
[dglt-2 /]#
-you are now chrooted, and partitions mounted and now have control of your already installed manjaro. then do the following:
pacman -Syyu
pacman -S mtools os-prober
and install if they are not installed already, and then:
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=manjaro --recheck
sudo update-grub
exit
exit
reboot
if all goes well, your manjaro grub should come right up and you should be able to boot manjaro or windows. good luck, let me know if it works out