Earlier quoted context omitted.
Why not just run an OS in a VM and full screen it?
Because it's very cumbersome. Managing with multiple audio devices and displays must be done in the host OS. Keyboard shortcuts might not work correctly. Gestures might not work correctly. Suspending and resuming is a pain, basically need to login twice. It's a lot of friction.
I'd disable that as one of the first things. Just autologin with no password.
For the thing running in the VM, not the host.