I was not aware of the first two issues you mentioned as they were not noted setting details at the time. I have no idea what to do about the first one, but as for the second that does not prevent it from ever happening especially given she had no idea about the larger magical society until after she killed the robbers who killed her father. If I really have to change it she could have killed them while they were robbing her home I guess.

The logic behind her regressing herself to a child is that it makes it easier to move covertly and less magical energy is required to heal a smaller body(also children do heal more quickly actually). It also makes it harder for her to be recognized and not being taken seriously is an advantage at some times. She was fully intended to physically be basically useless and rely on her magic to keep up with the others. I will admit part of the OOC motivation is because I find the idea of a 200-year-old witch with the body of a child amusing but I have tried to justify it IC.

Charlotte was teaching her apprentices dark techniques, but she did not teach them true black magic and assumed that her teachings would make them follow her path. She was mistaken but she did not teach them black magic.