Jipperty,
1. Yes, flight deck doors are locked from the inside, and can only be opened by the flight crew inside. In the case cabin crew need access to the flight deck in an emergency like pilot incapacitation, there is an emergency code, which, if entered, will give a signal to the pilots, They can then visually check who is outside the door, and if it is not cabin crew, they can elect to keep the door locked, and there is no way to get through it.
Door guarding procedures are varied depending on airline policies and regulations, but yes, the cabin crew is instructed to try to stop anyone forcefully entering the flight deck in-flight. Whether they are able to or not, won't be known until it happens. I, for one, would do my very best!
2. Yes, as far as I am aware, airlines do have the duty of care towards all onboard. I wouldn't be sure of legal interpretations here, but generally speaking, yes. Airlines are also required to carry first aid and/ or medical kits, these are part of the so called MEL (minimum equipment list), without which that aeroplane cannot fly.
3. Im really not sure about statistics on that one, I have no idea how frequent or serious in-flight fires actually are. My understanding is that they are rare, but hey, that's exactly why cabin crew are trained to deal with fire and smoke scenarions, just in case of the rare event they do occur.
Equipment used for cabin service may, of course, be the most common reason for fires, however, that is not to say that without this equipment no fires would occur at all.
To sum up my opinion on why it is necessary to have cabin crew onboard, it is because the aircraft operates in a very specific environment, ie far above the ground. If you have a fire on the train, you do yuur best, save yourself then STOP AND CALL THE FIREBRIGADE.
If you have unruly passengers on the train, or someone causing criminal dmagae, or committing a crime, once again, YOU STOP AND CALL THE POLICE!
If you have any kind of medical emergency on a train, again, you do your best, STOP AND CALL THE AMBULANCE.
What do you do on a plane, though? You could carry a few firefighters, policemen, medical professionals, etc on every flight. That would be expensive and unreasonable, given that a serious incidents don't happen that often (nothing in my case in nearly five years of flying) It is a lot easier to employ the lovely cabin crew who are trained and practiced (to an extent) to deal with these scenarios, although only trained to the extent it is necessary (ie no, cabin crew are not fully trained fire fighters, policemen, nurses, doctors, terrorist squads, but a little tiny bit all of those).
B