Housing manufacturers do not get advanced previews of new camera models! The underwater market is so tiny compared to the camera market in general that we don't even warrant a thought.
Most of the time, the manufacturers use CAD to use try and estimate how the housings will be and prepare prototypes so that once the cameras ship they can ship quickly thereafter.
I guess that Subal may have one of their pros that did some pre production testing of the D5 and were able to get the crucial measurements off of it. Obviously, they have managed to get their hands on one of the very few Nikon prototype D5s! Predrag?
Button 4 down on the left hand side! The D500's is in the same position.
The D500/D5/ will give the option of (with suitable strobes) giving TTL with flash output control, via that rear button.
I am unsure what you are asking:
How you would even be able to link an underwater strobe to the camera and shoot TTL may present new problems to the strobe makers since as far as I am aware, radio signals do not transmit well underwater.
The D500 can trigger strobes electrically and via the many electro-optical converters that exist. Some of these do not work via TTL, so this may present a challenge given your shooting style.
Your slave strobes, although they are wireless, are actually being triggered optically