Having chased similar issues in the past, I'm offering up my thoughts on the matter; I may be telling you many things that you already know and if so I apologize in advance.
-- I have often been irritated at changing frames while zoomed. It's as if it looks at the shift required to get to the next frame, THEN rotates it, THEN applies the shift without accounting for the rotation. It's not just you. The changing rotations you observe are probably because Vuescan remembers the last rotation for each frame and assumes you want to keep it with the next preview. So I often (last night, in fact) find myself zooming all the way out and stepping through the frames one by one, setting the rotation as I go, before I even bother setting crop positions or white/black points. It still jumps to the wrong place in the preview area if I switch frames while zoomed, but at least the rotations are correct. Then I'll set the crop for each frame, and finally set white/black points and curves per frame if appropriate.
-- White and black points are set as a percentage of the input pixels that will be black or white in the output image. If you go from a contrasty frame with the sun and some black velvet to a full-frame shot of a gray card and keep the same W/B points, you'll get the same number of white and black pixels in each frame - which you probably didn't want and it'll look very odd.
-- Fortunately, Vuescan remembers the white/black points for each frame separately (unless you select the "All frames" option in the Color tab), so you don't have to have that problem... but if the crop area is offset for some reason, it will calculate W/B points over the incorrect region, possibly including the unexposed edge or the completely exposed leader, etc. This will throw off your output values and look all weird and stuff.
-- You CAN set "Lock Image Color" and then the white and black points ARE absolute values within [0 1] independently per color, rather than percentages, and they don't change from frame to frame. But note: if you're scanning film, you'll have to uncheck "Lock exposure", do a Preview pass, check "Lock exposure", Preview again, "Lock Film Base Color", and then you can set "Lock Image Color." Don't skip any of the Preview steps. For these steps to work correctly, the crop region during these steps should include min and max densities (e.g. unexposed film edge and exposed leader) so that Vuescan can see what are, effectively, the white and black points of the film response. Again, if I'm telling you things you already know how to do, I apologize. Point is, "Lock Image Color" will prevent those W/B point shifts between frames that you've described.
-- My workflow doesn't really need it, but I believe that if you scan at 48 bpp and output to 48 bpp, the W/B points and color balance, etc. aren't applied to the output image. Others may want to chime in on that. I always scan at 48 but output to 24, because none of my work is yet good enough to worry about better than 24 bpp. But I've still got the negs/trannies...
