Another 2 cents from a relativly novice view.
... They look the same, but appearances are deceiving, IMO. Ikkyo controls center through the elbow, nikyo controls center through the joint lock at the wrist (not pain).
This is also how I always understood the differences b/w Ikkyo and Nikyo. My understanding is that if the left hand control the elbow and the right hand just grabing uke's wrist
while doing the sabaki either Omote or ura .... then it's an Ikkyo technique. If, on the other hand, the right hand is twisting and locking the uke's hand & wrist
, then it's a Nikyo.
I always thought the ground pinning at the end was the conclusion to the technique (or part of it) and not the technique itself.