I'd like to throw in my two cents on the issue of uke teaching nage. In our dojo it is generally discouraged for an aikidoka to "teach" his/her partner on the mat. We call it shadow teaching and there are several reasons for this position. First, there is already a teacher (Sensei) on the mat. To presume that the Sensei cannot attend to those who need help is the height of disrespect and arrogance.
Secondly, constant instruction from uke can be confusing to nage especially if they are junior and want to be respectful. But, everyone learns differently if nage is thinking about his/her center and uke is telling him/her to change his/her feet then the training process is disrupted and the learning curve becomes steeper. Even a Sensei who is constantly interrupting his/her students to correct them can cause frustration and prevent learning.
Third, if uke feels free to discuss the technique with nage and the whole dojo is doing the same training grinds to a halt. I've seen classes where everyone is talking and no one is training. What a waste that is.
So, in our dojo we suggest that training continue with a minimum of talking. If nage is completely lost uke can guide nage through the technique but, ideally, without discussing nage's failings. Other than that nage is charged with discovering the technique as it is appropriate to his/her physical capabilities.