To answer your questions.
1. No, I don't think they started transitioning before puberty.
2. No. I am not aware of anyone who started transitioning before puberty reaching the elite level.
3. I don't know of any research on the potential advantage of trans females who started medical transition before puberty. There are not many of them engaged in sports, and even fewer at any competitive level. It would be hard to conduct such research.
4. Puberty blocker usually suppresses the t-level to "normal female range." It is usually followed by estrogen supplement to induce female puberty.