I saw recently in the Even Shelema which is a collection of statements from the Gra that even when you know the person will not accept your rebuke. I seem to recall the Shelah say the same thing. On the other hand one does need to know how to deliver rebuke. If one can not do it without anger then certainly he is not obligated. And the most obvious point is one does need to know when something is truly wrong or not.

There are good arguments for free will. Michael Huemer presented a few and so did Maimonides. But in any case once there is free will you have to assume that men and women can choose good or evil. The only moral superiority anyone has is when they choose good. That is clearly what the Torah holds.
On a side note there is a command in the Torah to rebuke your fellow man or woman when you see them doing wrong.