My take on this phenomenon: Yukkuri's memories are in their bean paste. When Reimu defecated, she managed to somehow rid herself of her nice qualities along with the expiring bean paste, hence the gradual progression from nicehead to shithead.
^ Weren't you the one who would let them go scot-free based on that one polite response?
Yes, take them in as pets. If they turn out to be shitheads, it's not like their owner can't do anything about it. There's plenty of options, from discipline training to downright abuse.
They had forfeited their freedom the moment they acknowledged Anon as their benefactor.
^Yukkuri are shabby ripoffs of Touhou Project characters and weak monster fauna mobs in video games that are also iconic mascots (slimes in DQ, porings in RO, FOEs, rabites in Mana series, etc.).
So this story is rather appropriate. I'm actually expecting more fairie tale stories of a similar vein, if they haven't existed yet.
I might even write some up if I ever had the inspiration to...