I thought IMAX used a specific (rather narrow 2.5") interaxial fixed separation in their 3D camera. Everything I've seen in their film productions uses that basic IMAX 3D camera. I have never seen any variable IA IMAX cameras used for Hollywood films. Didn't think they existed.
So, how it was shot is always the same IA. Not saying they didn't do something in post because we all know those tools exist to mess with the screen position.
There are systems in use for 35mm and various digital including RED cameras that work on a variable IA bench. And maybe you saw one of the IMAX experimental tests that used two IMAX single cameras in very wide separation for a test but I don't recall this ever being used in any commercial movie.
As for changing an IA in post or simulating that, by conventional means, that would be impossible because when we shoot real world twin cameras from two different locations, even the 3/4" Bloggie 3D we are seeing the same object from two locations at the same time.
In otherwords, it's like being in two different places at the same time. Theoretical physics aka quantum mechanics says this is possible for one camera to be in two different places at the same time but nobody has yet achieved real world QM capability.
So, there is no way to simulate interocular camera separation with one camera presently. And, there are also those theoretical physicists who don't believe quantum mechanics is possible either, aka string theory or parallel universes. By suggesting that a single camera view can photograph a scene from two different places at the same time requires the use of this new area of physics. So, presently we can only shoot a scene from two different locations at the same time by using two cameras.
What the parallax or horizontal separation adjustment of the two images does in post is not change the 2 camera perspectives but just change the viewing screen plane along the z axis location. The only way to change the size of the z-axis stage is to change the interaxial camera separation.
Bottom line, I don't believe we'll ever have the capability to make true stereoscopic movie from a single camera as it violates conventional laws of physics. It is possible to freeze the scene, move the camera and take a second image from a different location later in time but this can only be done by freezing the scene in time, therefore not possible in movies, only good for still photos.
Regarding your editing test- I think your demonstration will work as long as there are no extremely close objects sticking in the scene when the overall scene extends into the great distance. I think you had one shot like that in your iso where the lower part of the scene was extreme negative parallax and the upper half was extreme negative. My eyes were adjusting greatly to take in the lower part and then again for the upper part but to view the whole image at the same time became quite a challenge. You may be able to adjust the horizontal separation and place the lower part of that scene at the screen plain. Only testing will tell. I had the same issue with my extreme IA of 26" with the water of the river extending way in front of the TV and the buildings and sky over a mile away way behind the TV. Moving the entire scene into positive parallax helped.