does your D810 fit in the housing without the back on? Can you get the base plate to align as it should be? Or does you problem start there? I have to push mine a half millimeter in on the right side, but very gentle. And after that the housing back fits easy...
kind regards // Alex
"Perseverance pays off." Thanks, Alex, for your continued help. I'm happy to report that I have now acquired all of the functionality that you originally reported!
My base plate with camera mounted fit snug and flush in the mount, but your suggestion of playing around with this got me to lock the base plate without it seated all the way in…i.e., I let the camera sit ~2mm closer to the back. This allowed the buttons on the back to finally reach far enough to depress the camera buttons enough. So then I made a temporary spacer out of cardboard to keep the base plate from going all the way in, which continued to allow the buttons to work. But I noticed that clamping the back down didn't appear to seal the bottom side of the housing; I could see the O-ring from outside. Looking carefully it became apparent that the photo/video switch is what protrudes the furthest from the back, and since it's now misaligned with the new position of the switch on the camera it must be the cause of the insufficient seating. Appeared to be easy to remove the switch from the inside of the housing, a single Phillips screw, so I did. Replacing the back still left a gap on the bottom, so then I tried it again without my cardboard spacer. With the base plate seated fully in the mount all the buttons now work and the back appears to seal fully! The photo/video switch couldn't be used anyway due to the misalignment, so removing it from the inside of the housing (which still leaves the switch sealed on the outside) seems to be a solution for making the housing compatible with the D810 and retaining nearly all the button functionality.
Have you tested this in the water yet? I guess I will now (tub of water test first).
While some users may be fine with losing the capability to record video, I actually am still determined to sort it out, and I think it's possible. The photo/video switch actually wouldn't be necessary. I just tested what happens if the switch is set on video and you hit the shutter button…The camera still shoots a photo (you have to specify this in the custom menu). In fact, the camera acts entirely in photo mode unless you hit the Live View button to start video mode. So I just need to figure out how to get the arms to reach the Live View and Record buttons. These buttons are within millimeters away from the existing housing ones, so it seems feasible. Still interested to hear any guidance on this if anyone has any or can direct me to some!
I came very close to ordering the Nauticam. But I think I'll be able to stick with trusty Sea & Sea now!