You can see how James Cameron paid careful attention to depth of field in Avatar. If the foreground or background are blurry in a 3D film it seems to cause confusion as you can't focus on these things even though, given it's 3D, you might think you should be able to.
It's theoretically possible to produce some kind of hologram-like 3D that would side-step this problem.
The focus issue is related to the focus of the camera that shot the film in the first place.
It is not. The eyes need to focus on the cinema screen, this has nothing to do with the camera. The problem is that the eyes need to keep the focus at the same distance, while the converging changes distance.
I'm not sure how it would work, but the idea is to change the dioptry to the inverse of the distance of what's supposedly on the screen, so that our eyes need to focus at the same distance as the converging, effectively nullifying the inverse focus.