Without a little insight into the PRE end of the world, hard to say much about what's going on now...
The Zombie wiki has the Solanum explanation specifically related to decomposition.
The toxicity level of undead flesh, combined with the 100% fatality rate for infected lifeforms significantly slows down the process of decomposition. This is because all lifeforms, even the bacteria that plays a role in natural decomposition is also killed when in contact with Solanum. This has the noteworthy side-effect of making many zombies practically odorless, due to the absence of the bacteria responsible for body odor and the traditional smell of rotting flesh. Zombies that have been undead for many months may start to show minor signs of topical decay, and the recently turned may smell because humans void their bowels after death. Other than that, zombies produce no odor.
Then how does this explain Michonne having her two pets to keep the other zombies away? I thought they were basically covering her scent.
As for the episode...
Fantastic! From the prisoners' ignorance, to their rioting attack, to our guys' "didn't we just tell them...," to Rick settling things with their alpha, just great. I did feel a bit bad for the two left, but that's what happens.
I believe Tiny got majorly scratched by the arm bone
of the zombie behind him after said zombie broke off his own hand to get rid of the handcuff. Better at the start than later. And I agree, the younger kid was a follower who freaked when his alpha got killed so quickly and gruesomely in front of him.
So now our group has a "safe" place, some food, a bit of medical supplies, and still haven't hit the armory. They could set themselves up okay.
I think I'd leave a few zombies in the outer yard for protection, removing the bodies to not give things away. Most passing humans would not think that the prison was safe if they see zombies wandering outside it.