Transforming to a female can be of two forms: 1. If the person is really a female with feminine specifications like a womb and female reproduction organs but with some male characters and the transformation process is merely to strengthen her feminism and show its clear characters then the said person is actually a female and all applicable laws apply to her. 2. If the said person is actually a male with some feminine characters like fineness in voice or similar characters then it is not allowed to consider that person a female.
If you fear that your health will be harmed by fasting, you are not obligated to fast, and you should make it up later when you can.
Yes, it is permissible with the hope that such ghusl is desired by Almighty Allah.
You should try to advise them, and their bad actions do not affect you. Almighty Allah said: "And whosoever does an atom's weight of good will see it” (7:99).