Why does it bother you if Rav Chaim Ozer was wrong on this one issue.
It doesnt make him any less of a Gadol if he did not total understand what Hitler intended to do, Many people made that same mistake.
Would you respect him any less if he was wrong this one time. And he wasnt the only one. Many others gave similar bad advice.
Jews do not belive people are infallible, Only Catholics belive that.