1. For truly popular mangas, no serious group is going to wait for a chapter to be released in China or Taiwan[0]. Someone that lives in Japan will go out and buy it. Or wait a few days for the it to posted online for sale by the publisher (though sometimes most recent chapter will be free for a few days). It's all illegal so why would it matter if the raws were obtained in China/Taiwan vs. Japan/online anyways?
3-5. For many of the popular series, more "groups" than you would expect are really just one person. And perhaps irregularly a cleaner and/or a typesetter (eg KireiCake[1]). Some more "professional"/commercial groups have much larger teams, and even paid translators, but they tend to do porn (especially erotic BL), because that's where the money is at. I've never heard of pre-teens being part of these groups, but I guess no one would really advertise that they are a pre-teen. Shitty translations (or shitty MTLs) discouraging others is a real problem, especially for more niche series.
[0] Unless the Chinese language release is at the same time of the Japanese one? At least for moderately popular manga, I know the English release is usually months (or more commonly, years) behind. [1] https://www.reddit.com/r/manga/comments/1e1bkw9/a_statement_...