Why does the K668 Microphone not tilt forward?
Turn the screw counterclockwise and bend the microphone back and forth a couple of times to loosen it.
What is the threading of the K668 mount? Can it be attached to a boom arm?
The thread size of the mount is standard 5/8". To attach the mic onto a scissor arm stand with a 3/8" threading, you need to screw the 3/8" to 5/8" adapter to the arm stand. Such an adapter is usually pre-attached to the boom arm. The installation demo is as below (from 2:01 to 2:15).
Which shock mount and pop filter fit on K668?
There is no need for a shock mount, and no suitable shock mount for this microphone.
As for the pop filter, you can just find one on Amazon with the highest ranking or the most reviews. If you fancy getting a U-shaped pop filter, just buy one that fits a 48mm diameter microphone.