It seems like there's more time to rescue people than there was in the first Dead Rising, so I'm wondering if this is an actual possibility this time.

Yes but it is difficult to do during the first playthrough. Some of the psychos may be tough to beat with low levels. There are also some survivors that you need to pay (e.g., $100,000 for the 3 high stakes poker players).

Yep.

I think so but you need to be level 50 and have a load of money since you need to pay some of the survivors. Plus you practically have to use a FAQ when there are survivors that dont show up as missions. Or you could use our excellent tips and tricks thread.

If you're level 50, it's very do-able.

" If you're level 50, it's very do-able. "

With advanced knowledge of the unannounced survivors I was able to save everyone starting with a level 5 character from Case Zero.

You can do it on a "first run" from level 1. 
 
But you had to have played the game all the way through before and then wiped your save to get level 1.  You need to know where/how to find certain survivors ahead of time, you won't necessarily stumble over them in your initial playthrough so you need some knowledge of what to look for.

I'm 6 hours left till the military arrive and I've saved everyone apart from the woman you get for killing the CURE hippy, and that was only because she turned against me after trying to get zombies off her with a baseball bat.