Highlight and shadow are convenient shorthand for the effect of development. The whole film is affected proportionally, including the midtones. How could it be otherwise? The precise effect will depend on the actual materials used but Ian will know more (probably much more). This is something that has baffled beginners who have only read about the Zone system.
I've seen someone who cocktail-shook the tank and it worked perfectly well with her times, but she was advised that there was a danger of stripping the film off the spirals. I didn't see that happen.
Slow agitation on spirals (or other holders, I presume) means that the part of the film held next to the spiral gets a little river of developer running along it, clinging by surface tension, as you turn the tank, which counts as extra agitation and overdevelops the edge. I can't comment on rotary processing, but I assume that Jobo, et al have done their homework.
Developing in trays is another matter. Very slow agitation of the stack would give blotchy development. I don't see how you could do fast agitation.