You'll never get it to pure white in render, when I need that, I just add it in photoshop by adding a vignette, but going white instead of black, so the image gets pure white around the edges in a subtle fade.
Not in front of a computer, so guessing on the bottle issue: can you add a Corona compositing tag to the Shadow catcher geometry and hide it from Refraction? Can't remember if that's an option in that tag.
Or, render with the shadow catcher geo turned off, then render with it in and composite them together in photoshop.
e.