jcharles since you referenced me I thought I'd answer.
In order of your questions

1. I was told by Zygi that the single cap crossed the tweeter at 12K and of course that would be a 6db per octave slope.

2. The spec on the woofer is 6 inch but I've never seen it referenced. From frame edge to frame edge it measures about 6.5" and across the moving parts of the diaphragm including the surround measure about 5". So all in all quite stunning to hear what this speaker does with these two semi-diminutive drivers. Nominally it could be called a 6.

3. The cones are a specifically treated paper composite cone that have a black sheen that looks like they could be composite, but they are paper. The basket and frame are polymer and seem to me to be molded in one piece.

4. The frequency response is incredibly flat from about 120hz to 10Khz and has no spike in its gradual but precipitous upper roll off. IOW very smooth. The smooth natural upper end lets the tweeter mesh so nicely with this driver with only a single cap. In a cabinet like the MG944 the bass is brought around nicely from 200hz on down. It is a pretty incredible driver. So not exactly "holy grail" but a nice example of its category. The stated sensitivity is 89.9db/W@1 meter. Using two woofers the MG944 rated at 94db is accurate. The response gets a boost from cabinet and use of 2 drivers. SPL is limited and you can spank these speakers enough to ruin the sound and also destroy them, but the clean loudness I've gotten from these speakers from 15 watts is both brutal (in a good way) and very listenable. 28hz flat? Steve could explain that I can't. They play deep and tight.

5. Cone break-up? How's that handled with any cone driver run full range? Stiffness? Nelson Pass and his cronies have been playing around with Feastrex and other exotic paper/fiber cone full range (extended?) drivers of late in open baffle designs, and if he's not worried about it with 8" full rangers I'm thinking this little 5"/6" is not effected much.

5. The driver is easily available.

6. The 52" must be a typo. More like 42" tall. I'll measure mine.

7."True" quarter wave length tapered TL? is any tapered column of air captured by a solid boundary a true anything? There are horn behaviors and quarter wave resonances and bass reflex anomalies going on in many hybrid designs. Think of this
but folded in half and stuffed into the MG944. My explanation may be too simple but that is the general idea. Instead of a front "port" the big end terminates into a passive firing down onto the top of the plinth or "foot" of the MG944. Bass pressure is out the gap between the plinth and the cabinet. I'm not sure there is a technical answer to this question. I would not want to figure it out though. This speaker sounds good.

Mine are the original with the cleft "carlson slot" style cut at the front bottom and they measure 39". The plinth base type newer version might differ slightly but I don't have an example of that here. If you put the speaker on some large cone type spikes I imagine that could add an inch or two. Hope this helps.

As I said this speaker is a "no brainer". It is worth twice the price.

I been sayin Riv!

I recently spoke with ZYGI, and after my bass issues with the Torii/MG944s, he checked it out, finally deciding to reduce the MG944 plinth to radiator space. The current speakers have a 1/4 inch space for the passive radiator between the plinth and speaker box ....the same distance I arrived at with my experiments. Not that all my bass issues were the speakers and how the Torii's amazing bass pushed them, but it is cool that ZYGI got right to checking it out, just in case.

I love it how these guys are working to refine the gear exactly in time with noticing the possibility for improvements (no matter how subtle), and then they explore and refine to suit their long developed tastes as listeners and builders.

And back to this thread origins, how ZYGI got the bass he did out of this variation of transmission line speaker??? Well I don't know, but in my experience, these speakers have very deep, articulate and palpable bass....not dissimilar to having a sub in the system. To me, better really, because it is all designed into this same very balanced sounding package without unnecessary complications!

Lucky us!!!