1- Yes they are created from fire.
3- There is no evidence that they can take recognizable human figures (like taking the form of your brother or friend for example). Their origin is fire, now this doesn’t mean that now they have the properties of fire. We are created from clay, but it doesn’t mean our bodies behave like clay. So even though they were created from fire they can take on different forms because they are light and less dense than humans.
4- It is disputed among scholars. Some say yes while many say no they cannot possess human beings. Yes they can whisper to humans or cause them some harm but they cannot possess them and control them.
5- It is basically Du’as to protect for their influence. Those Du’as that have passed down from the Ahlulbayt (a) are trusted. Otherwise we don’t trust them.