I remain unconvinced that all photos taken with converging optical axes can be corrected. The very simple reason for this is that if the convergence is at a subject at close range, the image recorded at far range may be completely different due to the axes pointing at different parts of the scene that do not match.
In the situation you describe, I see the combination of two effects:
1) The keystone effect, due to the angle, which can be canceled out (with crop implications already mentioned)
2) The extremely excessive background disparity (going out of the image because the field of view is limited), due to the too large base, that cannot be corrected (at least not easily).
By the way, such mirror design could produce parallel shots if the angle of the mirrors were adjusted for that.
Could you detail how you come to this conclusion? I am not very good in optics (so my initial assumption might be wrong), but your version does not sound right to me.
I tried a quick and dirty simulation. It is very simplified, but it tends to confirm that the two situations are not equivalent. Maybe I missed something.
Also, I'd like to remind all that the subject of this thread is commercially available 3D lenses - not whether it is technically possible to correct distortions caused by poorly designed accessories.
My point is that if a product with inherent design flaws is put on the market, the buyers will mostly use them "as is" and that will result in images that cause eyestrain.
Sorry, but this sub-thread started because you suggested that stereo converters for single lenses shall be avoided because they introduce opposite keystone distortion with their design inherently bad which is a no-go. Several messages responded that well used, it could give interesting pictures.
Your arguments are not weak per se (people thinking or advertising that it gives good stereo without work is sad), but are weak in regard to the statement that the stereo converters for single lenses are inherently bad. I do not use them myself, but you would not convince me with this argument.
I perfectly understand how these distortions behave. This particular line was to generalize your argument that using a device that has distortion to correct the distortion is very stupid, and show its weakness because we do it with lenses all the time. I thought about 2D photography, but actually the distortion of the lenses introduce mismatch and thus 3D discomfort as well.