Ok, I get it with the back. The insert side-loads, right? I was thinking of my Bronicas, where the film rails are accessible with the insert out.
Regarding the image centering, I would say it is not absolute proof that the mirror angle is off. Maybe the focusing screen and the film gate are not centered to each other. I don't know what percentage of coverage of the image a Hasselblad screen gives, but lots of cameras are around 90- 92%, giving adjustment room.