Regarding the first question, optically I think there is no difference between the 42mm adapters and the C mount adapters.
The use of one type or another would be determined by availability and price, although in my opinion, mechanically and aesthetically, 42 mm adapters are better.
As for the second, in my assembly I do use a helical adapter with a variable extension between 17 and 31 mm. All my adapters are inexpensive, Chinese in origin, but of pretty good quality.
Specifically, the helical adapter works very smoothly, without harshness throughout its travel.
With this optical system I have recently digitized two complete reels of 120 m each without problems.
Once the focus was adjusted, I didn’t have to retouch it at any time.
It is true that the mechanical operation of my machine is very smooth. In my opinion, to avoid vibrations, it is essential to adjust the stepper motor to operate at very high resolution, with small microsteps. In my device I use 32 microsteps per step, 6400 microsteps per revolution, in this way the vibrations are minimal.
I take this opportunity to comment that the final assembly of my optical system is not the one that appears above in this same thread, but rather the one that appears in this other thread https://forums.kinograph.cc/t/my-telecine-machine/ 2054/9.
The reason for the change is the impossibility of focusing with the first montage.
If observed, with the first mount the distance between the lens and the sensor varies, but the distance between the lens and the film also varies, which prevents focus.
With the final mounting, the distance between the lens and the film remains constant, only the lens - sensor distance changes and with the helical adapter it is focused with great ease and precision.
Greetings.