Re: tightening the KP needle shank onto the cord threads: it's a matter of torque. Ask your husband about torque! As in changing a tire...just the right amount of torque is needed to secure the lug nuts.
It's the same with securing the KP needle tips to the cord threads.
BTW: I rarely use the "key" that's provided with the KP interchangeables. I mainly use a T-
pin that sits in my pin cushion. It works wonderfully! I have good leverage, too.
Both Jan and I use a little piece (1" x 1") of rubbery shelf liner to hold the needle shank in one hand while we 'torque' down on the key (in my case, T-pin) with the other hand.
It's a matter of just the right amount of torque. And I do it kinda quickly, with a little burst of energy.
Not so much torque that you strip the threads, but just enough to really anchor it.