Poskim, especially Ashkenazic ones, will argue on *halachah* with rishonim, even when they don’t have a competing rishon to back them up.
No. They don’t. Some achoaronim were willing to, but there is no contemporary posek who will. And even when an acharon did, it was not with the glee of; “and B”H he has been proven wrong”.
Kal v’chomer on a matter of empirical fact.
While I don’t think we need to believe everything any rishon said when it does not relate to halacha, it is hard to consider psychology a matter of empirical fact.