I disagree. Every single consumer 3d tech is a fixed focal length. Neither the 3d tv, nor the movie screen changes distance. there's nothing special in the glasses to change distance, it's simply blocking out images from one eye.

I think you mean something like the image needs to appear some distance away from the eye. Yes, i agree with that. There are old solutions to that problem.

You could show each eye a different image, and get a depth effect... but that has all of the problems you pointed out.