The 1/4 strength provides a more noticeable glow around highlights and a greater reduction in contrast than the 1/8, making it the most popular choice for a visible 'filmic' look.
No, the filter is made of high-quality optical glass and does not interfere with the phase-detection or contrast-detection autofocus systems of modern cameras.
Yes, you can stack it, but it is recommended to place the Pro-Mist filter closest to the lens to minimize potential ghosting or flares.