About people who give you neighborly help but you can't return it. I know how you feel about that! I still sometimes get help from people I can't return the help. I try to remember that they are getting a benefit from it (coins). I do neighborly help for most of my neighbors regularly, and only about 4 out of 20 help me, so chances are they won't even notice! One thing you could do is write them a message to thank them. It can be a good way to make friends. I found my current fellowship by writing a message thanking someone for taking my marble trade.
About the chests you can receive from giving neighborly help: Those are the rewards that pop up like relics or spells, You can get up to 3 of those a day, but you are not guaranteed to get 3, so if you only have 5 or 7 neighbors unlocked you might not get all of them in a day. Joining a fellowship can help because you can help everyone in your fellowship and with that many you are pretty much guaranteed to get your 3 daily chests. The rewards go straight either into your main hall (for relics) or into your enchantments inventory (for spells).