If everyone's infected, all it takes is for them to die through some other means. Heart attack, aneurysm, falling, starvation, etc. The scratches they saw could just be incidental.
There were no signs of death at all. Two young healthy guys aren't going to croak from natural causes like heart attack at the same time. They didn't have any trauma such as falling, and they didn't look starved. They might have committed suicide by taking poison, but a couple of armed sheriffs are more likely to shoot themselves in the head - the means is readily at hand - yet that didn't happen either.
The virus is what killed them, and it's going to happen to everyone eventually. And Rick is immune.
This is a good plot twist because it means that anyone can turn at any time, without warning. This will destroy the one element that they have going for them - social cohesion (such as it is) - and really ratchet up the stakes.
For people to turn after death doesn't change the stakes much at all. The most likely way any of these folks will die is by zombie bite, which means they're pretty much fated to have their brains blown out post-mortem now. So what if that becomes more of a necessity? It's a plot twist to nowhere. I hope this show is being written better than that.
As for Randall, how can he be able to walk at all, after that horrific leg injury he got? And if Rick thinks about it for ten seconds, he should realize that Randall is trustworthy and sincere in being willing to switch loyalties to them, because he's too stupid to be putting on an act. He spent all that time in the truck and never once asked himself "why are they bothering to drive me out to the ass-end of nowhere?" He should have realized that the last thing he should do, is let them know he's a local boy.