The keyring is important as it contains an update to the keys.
My old disabled key is removed and a new key has been added.
To avoid the random occasion where the pacman-mirrors package is updated before the keyring - you are advised to load the update package first.
Great - then the pacman-mirrors package should no longer be an issue.
But if you ran it on the live ISO only - then it is not worth anything
You need to correctly enter the chroot and if your filesystem is btrfs manjari-chroot -a will not work.
Please see this topic on how to correctly enter chroot when your file system is btrfs.
[root tip] [recovery] Basic Manjaro Linux Rescue and Recovery
In chroot - you need rebuild your mkinitcpio and rebuild your grub configuration
mkinitcpio -P
update-grub