And a punch on the arm is much easier to overlook than a slap.

But a punch is much harder to overlook than a suppressed action!

The point, had I been able to do it, would have been that my determinism-believing arguer had a POWERFUL intuition that I was the cause of his getting slapped

I agree; a punch would have caused perhaps a less powerful intuition, but it would have caused the intuition you desired nonetheless.