I am not saying you should reinstall - in fact any live linux can be used - the only thing you have to do is to mount the manjaro root partition from a live system.
Then copy the backup files back to undo the changes you made with the pacnew files.
The rough guide is
- boot a live Linux ISO
- open a root terminal
- mount your root partition
- if you are running a default EFI system the partition will most likely be /dev/sda2 or /dev/nvme0n1p2 (pure guess work)
List partitions
lsblk
Replace sdyX with the partition of your system
mount /dev/sdyX /mnt
Move into the etc folder of the mounted partition
cd /mnt/etc
compare the content of the backup files with their current equivalents
cat shadow
cat shadow-
and
cat passwd
cat passwd-
If you are confident the backups are ok
cp passwd- passwd
cp shadow- shadow
Move out of the partition and unmount the partition
cd
umount /mnt
Then remove the stick and reboot
reboot