First, K&F Concept ND2-ND400 lens filter allows changing shutter speed based on the effect you want with the best sensibility and the same aperture. Second, K&F Concept ND2-ND400 lens filter allows creating a blurred background with a large aperture while the shutter speed remains the same. Third, K&F Concept ND2-ND400 lens filter is designed to prevent overexposure and maintain image purity and quality at the same time. *Like a zoom lens, the variable nd filters are suitable for multiple shooting situations. If you're looking for perfect image quality, a fixed ND filter is recommended. 

Yes you can. 

This one is high quality materials, and contains both a variable ND filter + a polarizing filter. Also - most variable Nd filters show and “x” pattern when they’re turned to the max, essentially making them useless at full power. This one shows no “x” pattern. I’m happy I bought it! Very high quality and sturdy. 

For the noise you complaint on the microphone, we have tested many other microphone brand, knowing that all have this issue, and the noise is from the camera rather than microphone 

Unlike a tube it won't effect your focus distance nor effect your quality like a doubler. The lens will have more of an effect than anything else. On my Sony a7, I must be sure to use a full frame lens since the sensor is 35mm size