Well, I'll leave most of this (Notably calculating
) to the real macro experts, but where you start comparing magnification and focal length in that way, much of the real difference, regarding depth of field, especially, will be about *working distance.* Notice how that a 90mm achieves 1:1 magnification a lot further away from the subject than a 35 needs to be.
If you look at the focusing scale on a lens with depth of field scales on there, (like maybe that Tammie, if you have one, most old manual focus ones have these) you'll notice that as you focus closer to a subject, that the depth of field constitutes a shorter linear distance, (such as shown in those very short distances you cited) (for instance, there's a bigger space on the scale between two feet and three feet than there is between, say, ten feet and fifty.)
It's very like the difference between focal lengths outside of macro, just that the close distances exaggerate the same phenomena.
As for reversing lenses, as a rule you want the lens you've reversed to be shorter in FL than the one you mount it on. Zooms actually don't tend to be the best choices for this, but if i works, it works. You'd likely get better results with any ol' 28 or 50 that you can mount on there than with any zoom.