Charlie, you raise difficult questions, but I am not sure that they are all absolutes. As far as the Holocaust, because it happened so recently, and their are so many documents and photos and witnesses including Dwight Eisenhower who said that he is there to bear witness lest anybody falsify what he saw with his own eyes. Anybody who denies, obviously has an agenda of neo-Nazism or something similar.
But on things which are not as clear or as political, I think you can say what you like, but must bring good evidence, and explain the evidence to the contrary, as well. You may be looked upon as a wacko, in certain cases, but your use of the term “not allowed” I think is not the correct formulation.