If I were to attempt to aquire the CQB and Military Police helmets/variants would I be able to unlock them through the Master Chief Collection? An example could be finishing Halo 3 and ODST on Legendary. I doubt it but it’s worth asking.
No, you would need to do it on 360 due to you having to access Halo Waypoint.
As above - you need to do this on the 360 I’m afraid. For any other queries you can use the pinned thread 
https://www.halowaypoint.com/en-us/forums/cdecc00475c645b9a4d0a156fb542d59/topics/how-to-get-the-halo-waypoint-armors-for-halo-reach/814418ad-e302-4619-b604-d52ec9d7a69b/posts