6) Paralax can be consistently observed in real 3D display scene when you move and change your point of view, whereas the stereoscopic display lure the brain that there would be paralax effect if you move but, when you try to do so, it doesn't happen, you can't make a close object actualy translate faster than a distant one, and you won't see what's behind neither - not more that the other eye was already seeing.
Back to previous points:
#1) Yes, we can have all plans in focus at once for a stereoscopic display (like in video games), and it's more comfortable than not being able to focus on a plan that wasn't chosen to be displayed in focus.
But then there is still a difference for the user experience: when all points are in focus one is much more distracted by a distant background detail when locking at a close object. For a real 3D display the fact that only one plan can be in focus on observer's eyes retina is an advantage for the immersion of the observer and the depth feeling.
Yet, all-plans-in-focus-display can be an advantage for some specific purpose, like surveillance.
#3)
Most of the effect in #3 is mental, not physical. Your brain is not processing what you aren't focusing on (and no, focus is not an optical term in this context, if you know a better term for "location of attention", I'm open to it).
It's indeed a physical effect on your retina, due to optical eye property and geometry, where the really out of focus plan is completely blured, so the brain hasn't the information to process anymore.
Try again: focus on a detail 15cm from you and try to pay attention to a car one km away (almost align both): you won't even see that car.
Maybe some people can't do this experiment easily, but the popular blind spot experiment show that many people can center their gaze at a point while paying attention to another, you should be able to do so with some training and understand what I try to explain.
#4)
They add depth to width and height. Your argument is that 2+1 != 3. I don't think you'll win that one. That you object to the depth doesn't make it not there.
My argument is more like 2+0.5 != 3 , as you can read above.
Today's stereoscopic display only advantage on 2D picture is that eyes have a different perspective, that's nice but far from real 3D display. Focus issues, paralax, different perspectives for different observers (at once with the same device), ... there's so much missing to reach what real 3D display achieves.
I have a video game analogy, with some 2D games (Tetris), 3D games (Quake & many FPS), and 2.5D games. It's not about the display here, but the game immersion and possibilities.
#5)
And if you insist #5 isn't stereoscopy, then you are insisting that both eyes are fed the same image. I assert that's false.
The observer is not the display device. The fact that my eyes don't see the same image when I'm looking at an object doesn't mean that this object is stereoscopic. This isn't a property of the object.
The full holographic 3D display (Startrek one or other lab's device) doesn't make only two images available, one for each eye, but an infinity of images can be perceived, one for each place from which you can see it. Whereas stereoscopic devices present only two images, no matter what.
Also two spectators with the same real 3D display device will see the scene with a different perspective, while the stereoscopic "3D" movie show the same perspective for everyone.
I hope I haven't been trolled here and that I've been more than clear now, otherwise I won't know what I could add.