I think that the one-half wavelength criterion would be more than adequate were it not for the reflections from the ceiling and the floor. The little polar diagrams above you chart clearly suggest that this criterion should be adequate regardless. But I'm not sure those little polar diagrams are correct. Maybe it is for a specific crossover, but I'm not sure. Ordinarily, if the vertical spacing between the drivers is 1/2 wavelength, there will be a notch in the polar at +/- 90 degrees (for a phase coherent crossover where the lobe pattern is symmetrical above and below the horizontal). In the little polar diagram for lambda = 2D, there is no evidence at all of the notch that I think should be there.
" Ordinarily, if the vertical spacing between the drivers is 1/2 wavelength, there will be a notch in the polar at +/- 90 degrees"
Correct, I posted a ripple tank and source model links which show this is the way it is.
I was under the impression this was a DIY design thread and thought if it wasn't in stone yet, that they might model or investigate a non-traditional approach. What i don't see though are measurements of the drivers themselves as one is actually dealing with the magnitude and phase of the drivers and your adding the crossover mag and phase to them and for a passive crossover, you need the impedance's in your model too.