At one point this year, I also thought the key was to control uke with the hand that is on the forearm. However, this can result in simply using your arm strength to push uke to the ground (which is not aikido) or in collapsing the arm rather than locking it. The key is to (1) get correct maai with the hands in the correct position (2) step in without using the hands to manipulate anything and (3) stepping along the correct centre line. It doesn't work for me 100% of the time, either.
難しい , ね ?!!
As you said, the ability to use the technique in the painless manner de-emphasizing the lock that you described requires a cooperative / performing-roles type of ukemi. From a mechanical perspective, all three elements we've discussed -- the lock, the grab of the other wrist, and the step -- can be used in various proportions to achieve kuzushi and the desired result: uke falls.
Emphasizing the step and not relying on the mechanical advantage of the lock, to me, makes this an irimi-nage of sorts moreso than a nikkyo/nikkajo. In Hakkoryu we don't use these terms to describe this type of lock, but ultimately, taking into account the various opinions from aikido folks in this thread, a high-level practitioner should be able to execute the techniques along the same continuum as aikido: with excruciating pain (pain is expressly sought, for example, in the shodan-ge and nidan-ge that introduce variations of the technique and aspects thereof), no pain, or whatever's required in between to get the job done. That said, since the technique is introduced in suwari-waza (an example of which is in the photo in my previous post), stepping in would not be considered a prerequisite for executing the technique in any manner, pain notwithstanding.