In the second case -- it is completely other world. Usually one hand that controls attacker elbow during transition is not enough to really control countering attacker. I like to put attacker forearm between my hip and thigh to add additional leverage control with my weight before attempt to change from ikkyo to nikkyo. Then the way how you change has much less importance.
That's true... unless you are very, very fast, the attacker should be strong enough to break free with just one hand on the elbow
Tend to agree with Alec that transitioning from ikkyo to nikkyo doesn't make much sense if the ikkyo is already correct... maybe the wrist control becomes more important when uke is attacking fast with a knife... and nikkyo is applied using the hands only, no shoulder here
Also agree that transitioning makes more sense if the ikkyo control was insufficient.... unlike ikkyo, nikkyo (and sankyo/yonkyo) involves an added element of pain, which may be needed to regain control (which was lost when ikkyo control was insufficient or in error)
... interesting thread