All that said, many secondaries have a question about disciplinary action. Obviously I won't leave that blank, but I don't know what more I can say from what I said on the primary. Should I go into more detail or just rehash it? Should I try a different approach entirely?
Finally, would it be a bad idea to talk about the IA as a "biggest challenge" answer? Again, it was a very hard time for me, when I had to look at myself in the mirror and reevaluate myself. I came out of this process for the better, but it was difficult. I think that discussing that growth could be a compelling response, but also focusing so much on the most negative part of my application could be foolhardy.
They already know that the primary has a required essay for an IA so if they ask about it in the secondary, they want to hear more about it (or at least give you more chance to explain).
Was it your biggest challenge? If so, go for it. I can't say I would recommend all three in the same application, though