Could likely get used to it, but not in a few days. Once I turned it off, I felt a lot better.

I don't notice a focus change going from looking out the windshield to looking through the mirror. There almost certainly is one, but it's not as drastic, because you don't focus on the surface of the mirror... and, as you note, it's what I'm used to.

It makes sense. I wonder if a higher-res screen would also not have you focusing on the surface; shouldn't a high enough res screen be to your eyes just like a mirror? I'm just curious now why it's different!

I think you'd need a lens arrangement to make the image appear inside the mirror. It's not just related to binocular vision either, if you have focus a camera on an object appearing in a (flat) mirror, the focal distance is going to be pretty close to the distance from the camera to the mirror + the distance from the mirror to the object.

Now, I don't think you need to get this exactly right, but if you could make the video image appear at a focal depth close to to the length from the mirror to the back window, it would be a lot easier on the eyes than the image appearing at a focal depth of about a foot. I'm not good with optics though, so don't ask me how to do that! :D