The answer is that if the technique is being done correctly, both physically and kinesthetically, which we do not separate, it will Not cause pain. It's not about Uke responding to what Nage is doing in that sense. The pain only comes from incorrect execution (in my dojo.) Proper Kuzushi occurs, without pain, and Uke, in essence, has no choice about it.
In that sense, the onus is indeed on Nage.
If someone really continues to consciously hurt people, which has never happened, they would not be allowed to practice at my dojo.
I just have to say again, there is no way that Uke would screech if Nage does a good waza correctly. Without "understanding that" there's no real way to explain this way of practicing/learning Aikido. Pain is not a factor in our Aikido waza. Now, this isn't to say that we don't all make mistakes. In that sense, in that situation, Uke is responsible for his own safety, and because the possibility for this is always present, Uke is therefore "always responsible" for his own safety.
I don't know how to answer this exactly, we have different experiences. Have you ever trained in a good, solid Ki Society dojo where this stuff was taught? Maybe so, I don't know. That being said, Sankyo may be the "most difficult" to "counter" with Ki, but by no means impossible. Nikkyo, Yonkyo, Kotegaeshi…. all "not too dificult." This was one of Tohei's points early on about Aikido practice - "Attack with Ki" and maintain "one's integrity" which is what he experienced from O Sensei, and "regular execution of technique" does not work. I'm paraphrasing. Read his early writings about his experiences with O Sensei, other instructors and deshi, and how he formulated his approach to Aikido. Some of the answers are there.
I don't teach any technique that goes against a joint. As far as I know most "Ki-oriented" dojo don't, but I really don't know anymore. They may very well still do chin stuff, as Tohei seemed to like that in his early days (you can see him taking Terry Dobson down that way), but again, I don't really know now. I don't do it for a few reasons…. partly because, again, with a certain "use of Ki" it isn't necessarily effective per se.
I'm always interested as well.
By the way, I am independant for a reason.