chroot

run commands, edit files, install or remove packages

... or a kernel, why not:

mount disk to rescue system, provide network

well, you need some kind of system to mount the other hard drive, most cloud / server provider do provide something

mount /dev/sda1 /mnt

with btrfs, do this:

mount -o subvol=@ /dev/md0 /mnt

bind mount filesystems

mount --bind /dev /mnt/dev
mount --bind /proc /mnt/proc
mount --bind /sys /mnt/sys
mount --bind /run /mnt/run

chroot into your original system

get in there!

chroot /mnt

now you can run your commands, edit files and do things like you'd do on your regular system. like removing a package:

apt-get purge linux-1234

afterwards, log out, umount the mounted things from above in reverse order and reboot.