The Makedonian phalanx -- why such depth?

[PMBardunias]

...again, I don't think you've properly envisaged what Xenophon/I have described. The normal/open order is used for marching and, once the Phalanx is deployed into line, the 'approach march' ( see e.g. the description of Issus earlier by Callisthenes), out of range of the enemy and usually screened by light infantry/psiloi. Once the lights are recalled, or fall back through the Phalanx, the order is given to 'close up'.The 'advance to contact'/charge is then carried out in close order into "the teeth of the enemy" at a distance of roughly 1-200 yards ( across the effective missile zone).Any veering to the right occurs at this time, as men edge closer to their companions shields, and the Commander, generally on the right flank, endeavours to avoid outflanking and edges right....

Alexander marched 40 Stades! His more advanced drill may well have allowed him to do this in opened order. The greeks at Cunaxa deployed from column, the normal way to move men over distance, into a battle line in close order and did not budge until the armies were 4 stadia apart! Xenophon's men could march all the way into battle in columns of Lochoi if they desired. There was no reason to switch their spacing after initial deployment to move such a small distance. The only reason to do so would be to allow light troops to move through the line, but since we know again from Cunaxa that even in close order they could somehow open repeatedly for chariots, they surely could open from close order for psiloi.

Once the lights are recalled, or fall back through the Phalanx, the order is given to 'close up'.The 'advance to contact'/charge is then carried out in close order into "the teeth of the enemy" at a distance of roughly 1-200 yards ( across the effective missile zone).Any veering to the right occurs at this time, as men edge closer to their companions shields

Clearly this is not what occurred at Mantinea. The Veering occurred prior to the charge or else there in no way Agis could attempt to counter it. The fact that they do veer to get under the shield of their neighbor only makes sense if they were close enough to do so!

Bunching is unlikely, partly because of the size of the shields, held flat across the body for maximum protection, which also serve as natural 'spacers', and because the 'bunching under fire' you refer to is a modern phenomenon, of troops spread widely seeking moral support from their fellow-soldiers - Greek hoplites are already as close as they can be.

Right, so they were in close order because trying to march in opened order, 6' spacing implies roughly 3' intervals between men since the man is roughly 3' wide given the apis, would leave room for them to bunch.