Him being a former heroin addict is completely besides the point. His opinions and arguments would still be bullshit if he were the most competent medical professional in the world.

Nice ad hominem fallacy. This is at its core like saying AOC is an incompetent politician because she was a bartender once. It does not matter for the validity of their opinions.

The book itself does not glorify fascism but rather shows a fascist future human society. Heinlein wasn't a fucking a Nazi and the "allure" of this weird future fascism is the point on which the reader is supposed to reflect.