Another go...
In reverse order:
Tango is not a Latin dance.
Argentina is in Latin America, and modern Argentine culture typifies that which defines Latin America. Argentine Tango is inescapably therefore a 'Latin' dance. What would you call it?
As I understand it, the "ballroom dance" ballroom posture is connected below the waist, and separated above it - to help create that classic "X-shape" you see in ballroom poses. Whereas the AT posture is connected above the waist, and separate below it.
Well you can perfectly well dance AT in open embrace with no body contact at all, and that is also true of ballroom (and I would say it is the norm in most social ballroom dancing). However, I would say that actual body contact from the hip to the diaphram is necessary to all high level ballroom dancing, although it is interestingly not even mentioned between the covers of authoritative texts such as Howard: Technique of Ballroom Dancing, 2007, so ultimately has to be optional. I would prefer to think in terms of a good ballroom hold looking like a Y (and certainly not an X) and the tango embrace like an A, but the 'slope' of the latter can vary very widely, and as the dance doesn't have an accepted standard technique, there can be no right or wrong.
However, the basic posture or poise of the individual dancers is much less different than you imply. This would be no bad starting point for a tango leader:
"Stand in a natural upright position with knees slightly flexed, body inclined forward from the feet, braced at the waist, with shoulders relaxed at normal level and with no tension in the chest, body weight over the balls of the feet with the feet flat." (Howard: as above).
The Y shape of the couple comes from the lady being poised backwards from the waist, which shouldn't be exaggerated. The lady's balance is the same as the man's, though: forward, and that forward balance is maintained even when moving backward.
This really isn't fundamentally different from a typical tango setup. It looks a little different, but it has the same object, and for the same reasons. All ballroom movement is led from the body, which always moves first. Tango is therefore very good for ballroom dancers, whether leader or follower.
I'm not ignoring them - I'm just saying that we shouldn't pay too much attention to what they say. But I'd still be interested in their justifications for their positions, if you have a link to post?
Many in the tango community would not give much credence to the views of the established teaching societies, but then very few are members of one, or would care to be. I can't provide a link to the texts, as they are not available online, but I can quote from printed matter.
ISTD publishes: 'Argentine Tango - Syllabus Notes for Pre-Bronze, Bronze, Silver & Gold'. My copy is undated, but I think its been around a few years. They say:
"Forward walks are usually taken with a heel lead, while all backward walks are ball flat (no raised heels)."
Nothing more, and no explanation.
IDTA publishes: 'Argentine Tango - A Guide to Basic & Beyond', 2009. They say:
" ... smoothly extend the leg along its own line by sliding it along the floor. ... The footwork for this step is first ball then toe as it reaches its full extension. Weight is then transferred to the moving leg lowering the heel to end with the foot flat. Note: Due to modern trends, musical phrasing and increase in body energy, heel flat is used on limited occasions. Ball flat remains to be the priority footwork."
IDTA seems to me to be more in line with 'traditional' Golden Age technique (as described by Christine Denniston, anyway - I wasn't there), and perhaps ISTD is more in keeping with a 'modern' approach. The modernists largely reinvented the dance thirty years after everyone else had stopped, so who knows which is better. We could probably agree that neither can be right or wrong.