I'd say let the them still have some sort of advanced power from their old civilization, but they have to be resourceful so they have to use cheap material and sources so it has basically the same function, but a roundabout design. So perhaps even more advanced tech takes an even longer time an hey haven't quite worked out how to get their resources to fit around it. I don't know why, but I'm thinking of some sort of secret power source hidden away that runs everything and now I envision the Death Star blowing up. 

 And I love all your ideas!! Probably those residing in the church help with the trading cause obviously, you can't do charity without still having a day job. So shelter and good for protecting the trade route or helping distribute rations out. That also gives me the idea that there is some brewing corruption within those with trade authority or those in charge of giving out rations. I like the idea that everyone has a sort of forced equality, but the gap is starting to widen a little because people fear that there either won't be enough to go around one day or that the war is fast approaching them and only the 'richer' will survive.

 I wuv the war orphans idea, make your guy question his morality whilst playing with the kids or watching the demon interact with the kids. Working like a human despite possible demonic urges. The superiors that would frown upon both of their actions. Perhaps eventually, the demon has to leave because he's afraid the citizens will be harmed if he stays and gets outed.

 Hey, perhaps it's during a fight near the town that the angel seeks refuge and then discovers the demon who avoids the angel like the plague, until the angel disguises himself and corners him. Then the angel stays a while to keep an eye on the demon and recover so they have a time period to interact and the angel has a reason to stay.

 Hmm, I have an idea for my demonic character and a few of it's abilities, how would heaven and he'll be fairing in the war?