1) The best thing you can do right now is to buy VIP for the mirror client.
2) Yes, no way to prevent it but the best way to deal with it would be to not put all of your eggs in one basket.
3) As soon as you log on OSRS with the OSBOT client your ip gets flagged so yes, it already is.
4) No your main won't get banned as long as you don't bot on it.
5) Mirror client in itself is the best anti-ban there is.