i think its a great game, re the survivours, the first time you play id say it is best not to save to many of them (mainly go for ones near to the control room etc).
But later on in the game, alothough it does get slightly harder, once your skill levels are up it will be easier to guide them to the security room.
One way ive seen some people do it is, play the game once, and inbetween missions mainly explore the mall finding different places etc. Once complete play it again this time inbetween missions rescuing survivors as once you complete the game you will know the mall, shortcuts etc for guiding the survivors back to saftey etc.