Well, you at least know me, at least a little bit by writing here ...
I don't think it's stupid, to increase the tolerance of pain. You get a lot more freedom, if you don't have to follow "every little pain".
About yonkyo: It's a tsubo for accupuncture and it depends on your throat, lungs ... how an whether it hurts more or less. If you have to rely on pain to move your partner you won't move a lot of people.
Did you ever try to move a drunken person, a mentaly disabled person or someone else who doesn't recognize the "stop" -signals by pain? It simply doesn't work.
Don't only listen: Communicate. His statements will change if you answer him.
If he drops it or let go, the uncontrolled knife may hurt you while falling.
Take his center, have atari, control him and take the weapon gently ouft of his hand. ...
No pain, just technique. It's more efficient. ;-)
I have been moved around by pain, and I have moved people around by pain. Actually, I can be moved rather easily by pain. I have also experienced nikkyos that would have broken my wrist if I didn't move (my teacher is kind of good in all kind of joint locks -a side effect of also being a 6th or 7th dan in jiu-jitsu probably). Let's just agree to disagree, shall we? (Then again, you appear to be a masochist
About the knife: the point is contolling (i.e. locking) the hand holding the knife so he does not let go. For instance: tsuki -with tanto- kote gaeshi: hold the hand with the knife with both your hand and point the weapon at uke (thus taking his balance). Then push his wrist down towards his centre. This hurts and can even break uke's wrist if uke decides to be uncooperative. It's not gentle (knife fights rarely are), but you're safe.
I still think it's impossible to do a painless nikkyo or kote gaeshi.