Yes, this sounds like a simpler solution - the major drawback/advantage would be that you'd need to insert the knob in just the right orientation so that the shaft end fits into the potentiometer's slot. Might be a bit fiddly. On the other hand, the poti would keep its state even when the knob is removed, and you wouldn't accidentally change the value when inserting the knob.
Just to nitpick the nitpicker: the knob's shaft needs to be longer than 15 mm, right? "3.5 mm" is the diameter, not the length of a small T(R)S jack.
Oops, yes, 3.5mm is the diameter. I was sleepy when I wrote that, and I knew it didn't seem right.
Good point about the orientation when inserting. I was thinking something like hex or torx or friction fit that would allow you to insert it without thinking about the orientation. But then that causes another problem: any visual markings on the knob aren't guaranteed to be in the right position. I'm not sure if there's a good way get the best of both worlds on that.
Something like this might work. When pushing the knob in, a spiral inside it rotates around the tongue that is attached to the potentiometer (or vice versa). Once it is fully pushed in, the groove in the spiral rests against the tongue, allowing the knob to turn the potentiometer. Would probably need a lot of fine-tuning to get all forces exactly right.
