Why say に向かって　instead of へ？ It sounds better. "The rocket flew to space might be a boring sentence to someone trying to describe something important, like a rocket. You may want to say "the rocket flow toward the universe" or something, right? Yeah it's harder to say, but it sounds better. Also へ doesn't have to mean from "Earth". I could be from anywhere to space. It just indicated toward that direction. So in some context you very well may want to say 地球から宇宙へ飛んでた。

When I said you have to see it, I meant you have to see it being used in a real setting as opposed to reading it in a text book with a few examples with only small example of context.

There is no problem with learning Japanese from manga or games. That's the worst thing I've ever heard anyone say. There are vast amounts of manga with all kinds of different situations and plots and stories. There are thousands of different stories with different people who speak different ways and in different dialects. If you read Rurouni Kenshin you'd learn really old and unusal terms, which are pretty much useless but great to know. If you read Dragon Ball you'd learn hillbilly speech. You could read something like Gantz or Hajime no Ippo to learn what a highschool kid might sound like. Aside from actually hanging out with Japanese people, manga is one of the best ways to study.

I'm sure all of your degrees of politeness can be found althroughout a variety of different mangas or games. On top of that, things like しまう, おく、or ちゃんと, which may be difficult to understand are better learned through hearing or reading them in context rather than studying them from a textbook. If they don't make sense to you right off the bat, as I said, it'd be good to ask a Japanese person from the websites I showed. Sure, you could learn them as words from a text book. But you wouldn't get a feeling of how they are applied or what kind of neuance they give off in specific situations.

Also, concrete knowledge of mechanics? You mean grammar? Useage? Thats also the kind of thing you learn from experience. Yeah. Its good to learn how to read Japanese and learn the function of particles before you dive into a manga or a conversation in Japanese, but if you rely solely on a textbook anywhere after that you won't make any real progress.