As I understand David's post, he is saying that Aikido practice is not "real" in terms of actual fighting, and that we should be conscious of this and practicing accordingly. This makes sense to me.
I believe the best thing to do is to treat practice like a math problem or science experiment. The teacher gives us a specific attack to work with and shows how he/she could deal with it. We then pair up with a partner and experiment.
Applying this to Cindy's situation, I think her uke is wrong. She takes her partner's balance and he takes advantage of the slow speed and moves his foot to the place where his balance is broken. It might help to encourage the science experiment metaphor. You can work on developing good rapport with your partners so you can say to them that he/she is only able to move that way due to the speed.
This problem doesn't affect senior students only because they have a lot of experience practicing slow and simple. I agree with David's post, but I think it would be more helpful to discuss practical things we can do to solve the problem.