It’s been a while since I tested a normal debian installation in live mode with full disk encryption. IIRC on jessie this combination did not work. Now on testing and stretch it works. You just need to append “boot=live plainroot” to the kernel command line. For debian stretch you additionally should add the “nofail” option to /etc/fstab for the swap and the boot partition. I also had to chmod the VM images to libvirt-qemu:libvirt:qemu. You can then install the Whonix VM images. I tested it with the recent KVM developers version. Before you boot the host os live you also need to install the grub-live package and dependencies in the Whonix VMs and set them to read only afterwards. Then you can boot the host as a live system. Of course you can also use other VMs.
So a workflow for the security/privacy minded user could be: configure the host and VMs to your needs, install updates from time to time in persistent mode. Switch to live mode e.g. while browsing. If you still need some kind of persistence like for saving files you could always attach an USB stick to the Workstation. Your host OS and VMs should always remain unchanged after a reboot. If you want to be sure you can either create a checksum of the image and/or use some storage device with hardware write protection.
Overall the setup would be similar to Tails at least from the amnesia side. While debian testing is not that fast with security updates some users could still consider using it for better hardware support.
Copying everything to RAM does currently not work and would require some patches to the live boot scripts. This option would also maybe make not that much sense since you would already need ~4GB RAM to just hold the a minimal debian host + Whonix VM installation.