1. What if a guest trips and falls in my room / bathroom and decides to blame me? Do I need to put up a "caution - wet floor" sign in the bathroom?
If you think there is a safety hazard of wet floors in the bathroom, then yes put up a warning sign.
2 - What if a guest gets really drunk and hurt themselves in my room and decides to blame me?
This would be the guests fault if their behaviour led to them hurting themselves.
3 - What if a guest brings back a "Lady of the Night" or use illegal substances in my room? How do I stop him / her and should I report him to the local authorities?
Install CCTV or similar to monitor use of your listing. Having house rules that say that only guests who have booked and paid can be at your listing. If you know the guest is taking drugs or bringing in prostitutes in your listing you can ask Airbnb to cancel the booking.
4 - What if a guest damages the things in my room and then gets electrocuted / hurt and then tries to blame me?
Why would guests be vulnerable to electrocution in your listing? Look up airbnb host protection insurance for cover on this.
5. Should I just get them to sign some kind of legal agreement / waiver form?
Guests already sign up to airbnb's T&Cs when they book.