While i'm not phil , i use the nauticam 45 deg on my nauticam house ,
I review mij pics thru the viewfinder , never thru the back-screen , with my eye site its not realy a good option to look at the backscreen.
You can change the window easy by yourself its locked in place by a o-ring as Phil writes one minute job , having said that for me it never comes of...
You cannot use the backscreen with a normal viewfinder , theres a big hole where the viewfinder goes thru, i take the orig window with me as back-up if i wanted to do that.
ps i love the nauticam its a little wider than the inon , i traded mine in for the nauticam one I see Bill writes that there is a adapter to mount the inon to the Nauticam house , a year ago i could not find such a adapter so check that out !!