What was the point of an "alignment scope" like Kikusui 5091, and can anything be done about it's dismal horizontal bandwidth?
You're in worse shape than you think.
It's true, you flip the image axes to take advantage of the higher bandwidth of the scope Y axis. You could then modify your raster scan so that one line goes up, then the next line goes down, then up, etc, in order to avoid the delays caused by the retrace time.
Your problem is the Z-axis modulation frequency, which is abysmal. It suggests that you can get a visible change in intensity in 10 usec. Problem is, a 256 x 256 raster will have 65,536 pixels, so you'll only get about 1.5 frames per second.
What this does very nicely is, indeed, a vector display. As long as you're not trying to display too many vectors per second.
If you go the vector route, be aware that for consistent trace brightness you'll need to manipulate your axis generator so that the spot velocity remains reasonably constant.