It's a bit of a tough one. On the one hand, you can't really discriminate. Hence why when we brief passengers, I tell them that if they feel capable of getting to an exit without impeding or slowing the egress of anyone else they are welcome to. If they have a carer, I explain the same to them and delegate the responsiblity to them.
Of course, if there is an ABP who is willing to volunteer his/her help, they would be the first I would go for in a preparation time permiiting, to pair them up.
If it all goes pear shaped, I certainly wont be risking my life for anyone elses more than absoultely necessary, and I would hope the rest of my crew would be of the same opinion. 1 life lost is not quite as bad as 2, and even with basic first aid training, we are more use to the injured on the ground, and assisting marshalling passengers. Great quote from the united FA who survuved the Siox City DC10 crash - when the fires to hot, water too deep or smoke too thick, save yourself.
It's not a nice scenario to imagine, let alone deal with im sure. Unfortunately there is no 'ideal' solution that is in the best interests of everyone - and it would be impossible to prepare for every possiblity.
As for the stretcher cases, well, I would fully expect them to have a medical professional of some sort travelling with them. If they didn't, and it came down to live or die inside the aircraft, i'd be throwing em out the door quick smart. If theres no hope of them surviving inside, theres at least a glimmer of hope if they try and get outside. I certainly wouldnt want any nurse hanging round on board a burning plane arguing the slide decent would kill them. Shed have my boot up her a*se quick as a flash.
Its a very very tough call - history has shown us time and time again, what can start out as a small fizzle of smoke can kill hundreds in minutes.