I don't have any ready to hand literature (or even know if there is any) on the subject of differences in perception of vertical Vs lateral reflections. I assumed that the angle of incidence of the reflected sound wave at the pinnae would have a bearing on this & the idea that vertical reflections would be presented to both ears without the HRTF of horizontal waves? (btw, I think the distortions of the waveforms introduced by the pinnae may have a significant role to play in recognising vertical direction from horizontal direction reflections)Can of worms was maybe not the appropriate term. This issue would require some extensive digging in the scientific literature, but should you have an archive well filled in heat respect, give it a shot.
I was surprised by your "can of worms" phrase about this & reckoned I must be missing some obvious piece of knowledge related to this (which wouldn't be a first) & I was being too simplistic & naive in my statement?
BTW, I still have to read the links you gave to research in the field on the other thread so I may well be missing some fundamental knowledge?