I think that the reason that there is a significant amount of unused film at each end is to give some margin for:
Variation of where the film is actually attached to the backing paper, I've not actually check how much variation there is and there's probably less now than there was when the standard was layed down.
Differences between cameras, particularly those with mechanically-controlled advance (rather than using the red window).
I've not done any measurements but I don't think there's enough margin to get 9 6x9 frames, 17 6x4.5 is possible as I discovered when I mistook the "-" at the attachment region for a "1" while using a Super- Ikonta in 645 mode, the second of the two extra frames was on the film the first wasn't.
I'm not quite sure what you are planning, something like the overlapping panoramas that J.M. Golding often does with 127 cameras?
I think the direction is the other way, i.e take the left end of the scene first, then move to the right. Remember the image on the film is inverted, so the right edge of the scene is at the left edge of the film, which will then move to the right edge of the next frame on the film.