There is no way to know if you are applying Occam's razor correctly because we always have invisible cultural assumptions that are hard to escape.

Relevant story: my mother grew up in the Soviet Block where they taught her about American Segregation in elementary school. She said she and all her friends immediately dismissed it as made-up propaganda

In that case she was wrong. But I think the intuition is the correct "rule of thumb" to take. By your application of Occam's razor, you would end up believing most propaganda the Soviet education system pushed as long as it offered a simpler explanation. I don't think that's a good intuition to have either.