As I understand it, Apple is expected to only introduce a VR headset this year, like the new Sony VR headset or the high end Oculus. Apple’s AR headset is still many years away.
Apples definition of MR (Merged Reality) is "AR" (Augmented Reality) through cameras in a VR (Virtual Reality) interface. That means that it fully covers the eyes and can minimize the issues with outdoor lighting levels and precise alignment of projection over reality (If you move your head quickly AR will always be behind reality while camera images can be overlaid with minimal constant delay).
Mixed reality is a VR headset with cameras that pass through an approximation of outside reality. The hope is that gets you most of the vision benefits of AR. But you still get most of the physical drawbacks of VR.
All the current high-end VR headsets have pass through cameras, of varying quality. It’s not clear yet whether Apple is bringing anything significantly new to the table. Will somewhat higher quality components make a significant difference?