lol don't you love the notion of tachyons? They blatantly violate causality, you could essentially send a warning to yourself in the past via tachyon emissions if you mess up. Of course, there might a time range interval of adjusting your actions with foresight depending on how much faster than light they really are and the matter of if they spawn new timelines or affect the same timeline.
GF, I wouldn't generalize the external/internal distinction because I mean, if you think about, you could extend the internal aspect to be a matter of biology and genetic evolution, if I'm understanding your distinction correctly which you can feel free to correct. I think the difference is mostly in the explanation and that the explanation has some basis in our current science (however falsely) that isn't too far removed from the science. I mean, see what you tried to do with your Harry Potter example, you tried to ground it in our science. Think that's what mostly contributes to a separation of sci-fi and magic.
Also, other things of separation might be the usage of tech, shiny metal things and circuits and all those pretty little structured things, rather than the use of (sorry can't think of a better word for this) occult? instruments like piece of wood, animal/plant life that doesn't exist in our biosphere, etc.
There's a thing known as a vague predicate in philosophy. Basically, it's about certain words that have unclear borderlines. For example, the word 'heap' has an unclear borderline. If you have a hundred grains of rice, is that a heap? Is it still a heap with 99 grains, 98, etc. I think it kind of applies here in that if you have this much science in some work in question and this much unexplained phenomena (or magic) in the work in question, what is the borderline between how much of science vs how much of magic defines something as sci-fi or magic? And I'd consider that distinction to be something like a vague predicate, and additionally, that borderline, so to speak, would be different from person to person.