The Hoya Variable Density II ND Filter (1.5 to 9-Stop ) does not have front filter threads.

Yes, the knob on the Hoya Variable Density II Filter is removable.

Even with the previous VD filter i used the lens hood which would mount on the lens itself. In this case you will have to remove the screw, connect the filter and use the hood directly on the lens lock.

No, the metal handle is only for adjusting the strength of the Hoya Variable Density II ND Filter (82mm, 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.

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.

Yes it actually comes with the screw already removed, so you can choose whether to attached it or not.

I can confirm that I am able to use this Hoya II filter WITH the lens hood on my Olympus 40-150mm f2.8 PRO