Hoya Variable Density II ND Filter (72mm, 1.5 to 9-Stop )

Don't stack additional filters with a VND. It just introduces more reflective surfaces in the optical path. Since the VND itself supplies the lens protection that most UV filters are employed for, there is no upside to stacking multiple filters. As for IR pollution, be prepared to do your own testing. There are IR cut VND filters out there, but my experience with IR ND has not been good, even after purchasing a set from a major manufacturer that was endorsed by a famous cinematographer. While IR pollution may have been mitigated, there was significant impact to natural color in the visible parts of the spectrum, a weird color cast that is difficult or impossible to retouch or edit because it seems non-linear. This may be specific to my particular camera sensor, but if you go for IR filtration, you should test extensively before major projects where weird color renditions would create problems.

Yes the knobs on the Hoya Variable Density II Filter are removable.

Forget using lens caps with variable ND filters. By necessity, these filters are already thicker than most filters because they contain two pieces of glass plus a rotating mechanism to vary the angle of the two filters. This thickness introduces more likelihood of vignetting, especially with wide angle lenses or wide zooms, so most VND filters do not have front filter threads in order to keep the filter depth as short as possible. Do yourself a favor and buy a LensCoat Hoodie Lens Hood Cover to protect your VND and lens. The lens hoodie is a ton more reliable and never gets knocks off like normal clip on lens caps.

The primary use of VND filters is shooting video on mirrorless cameras, and many cinema cameras which do not have built-in ND filters. Click stops would make this filter useless for video. Most VND filters have indicators on the filter rings, but these are approximate. The one thing you should avoid, and the only case where hard stops would be welcome, is at the maximum density value. This is because VND filters contain two polarizers, and the planes of polarization eventually cause moire patterns. Very few VND filters contain hard stops, however. My guess is that doing so would dramatically increase cost because each individual filter would require precision calibration and markings, which would probably put a multiplier on manufacturing labor costs.

Hoya 77mm Variable Density II Filter gets in the way of the original lens hood and it cannot be used.