Why do so many Republicans seem to have such a problem understanding communism vs fascism?

Great 'question', although its more like a discussion. Anyway, I think you failed to pinpoint another failing of communism: centralized power structure, ala a dictatorship. I like to view good systems of governance like the human brain, the individual components (neurons, aka people) are quite incapable and frankly dumb on their own, yet when combined they can do more. They are collectively intelligent, and individually incapable. Communism doesn't follow this philosophy, it has centralized power structure which I believe is the core of why it fails and always will. The weakest link in the chain is the leader, get a bad leader and it becomes a cancer. Our forefathers were wise to create a style of government where power was decentralized, although this has changed over time.