Of the companions i have had so far they simply seem immortal. Is this the case or can some of the hirelings etc die?

Your the only person who can kill them I think.

Nope. They get knocked down, and if hit again while down before they recover they are deader than a doornail. I've lost two already. Definitely mortal.

depends on what difficultly but yes the can be killed it depends on circumstances though when they get knock down their aggro goes to zero and enemies leave them alone but if they are in an animation attack( say when dragon breaths)they will die or poison attack try it next you face a dragon let it knock down your friend then run beside him or her when dragon breaths they'll die