Just like any other animal, they drink water to keep them hydrated. When snakes are kept starved for days and offered milk, they do drink to keep them hydrated. They are cold-blooded reptiles. Forcing them to drink milk could sometimes kill them.
Just like other animals or us, they are made of cells & muscles. There is no such diamond or any other precious stone in their body. They cannot hypnotise anyone. Lots of snakes are getting killed for these reasons.
Snake's brain is not that developed. They chase only their prey's not anyone else. Infact their eyesight comparatively poor. They will not harm unless they are triggered.
They try to replicate the movement of their prey or anything that think is a potential threat, only when those objects are in their visible range. They don't have ears. They can sense vibrations. Snake charmers use these facts to their benefit.
No. Snakes donot bite at the first instance. Sometimes their bite is accidental. Not all snakes have toxic venoms. It is said India has abt 270 snake species &60 are venomous. If treated on time with anti-venoms, life can be saved.
Unless if there is any genetic deformity no snake can have multiple heads. At max two heads, which are common in reptiles, caused by genetic disorder in rare cases. Never confuse mythological characters with reality.