In the UK, and in countries with related systems of tort law, you couldn't be sued for not helping out. A doctor does not owe a legal duty of care to fellow pax until he answers the call for help.

If you did help out then you could only be sued if you were negligent and damage resulted. A court would take into account the surrounding circumstances - eg the fact that you were without equipment, nurses etc. If you've had a drink or two and this affected your judgement this still wouldn't be a problem if it was reasonable to assume that the patient was still better off with you after a couple of drinks than a sober non-medic.

There is no specific legal protection for samaritans but in the context of a flight you would have to mess up pretty badly - perhaps negligently to the point of recklessness before a court would entertain a claim.

One doctor told me that some airlines were asking him to sign a form before treating pax. He always refused - he believed it to be some kind of indemnity form saying that he understood that were willing to do an emergency landing and were reliant on his advice as to whether to do so. He believed it was to provide a defence should the pax try and sue the airline for not doing an emergency landing - they could claim that they were acting on the advice of a doctor.