Well, the 100 blocks away thing, were they in a more than 20x20 area? And it is actually elastic that multiple chickens spawn, they can be twins, triplets, and so on. I have experienced the light glitch... How far away were you? Were there any glitch machines around?
At first farmer's (brown robe dude) didn't plant crops, so went into creative and kept spawning a villagers and giving seed's til I found that would plant and harvest wheat which was a 1 fisherman which makes no sense
So went back on my survival to breed some villagers manage to get 2 shepherd to plant and harvest and was happy,although a lot of baby villagers died!! :'(. save game and logout for the night.
Next day reload world to find that my shepherd's have stop planting and harvesting. (Not a happy trooper)
Found I had to reload my world non stop til he started farming again.
So back to creative I went and spawn villagers til I got a brown robe dude, gave him a seeds. he didn't plant and harvest so saved world and reload til he did.
Surely it should be farmers that replant and harvest and not random brown robe villagers.
Actually, every update all the seeds change so they can generate the world properly. If you look on YouTube for "TU1 Showcase" you will see how different the worlds are and the generation. For example, the Extreme Hills Biome was more common and now that we have emeralds and trading they have to redo all world generation. See what I mean?