The gear knob is not threaded. The gear knob is pressed on to the shaft. Takes some effort to remove.
Your second question, my clutch has been replaced, the garage managed to replace it, with the engine in situ.
There are threads on removing the knob. It requires a certain amount of swearing, brute force and closed eyes to do ....... or a stanley knife, baby hacksaw and a LOT of patience.....search the forum and good luck!