"Play against the odds."
When Argan heard that sentence, he immediately understood what his lady was insinuating. The percentage of them succeeding in forcefully manipulating him was low, but they didn't have much choice since trusting the Autumn family would be risky. So, if both choices ended up bad, then why not do something that might possibly leave them an opening?
Yes, the principal could very likely go crazy if he was determined to defend against them. That would mean they had to break his mentality altogether in order to successfully compel his mind and the worst case scenario from all of this would be a mental disability. That would cause them a little bit of problem but wasn't something that Kalliope couldn't handle.