Thus there can be mental pain from untrue infamy, right?
That may well be the point.
There can be pain from infamy, but necessarily so? On the other hand, pain is always painful.
You're right this is good food for thought for the discussion.
It's likely that what we have here is Epicurus making a very legitimate point about something, and Cicero distorting it by taking it out of context, so our job is to unwind the problem.