I’m getting a bit confused now what happened when 
the guide book is coming in handy!
To get the details we’re talking about I needed a lot of lookups at exactly all those story points when I read it the first time. And my memory was fuzzy so I just went back to spot check a few things. Overall you’ve got it right and I’ve just put in some of the major detail to help.
I’m trying to keep it spoiler light. I don’t say on any one topic if more is coming or which parts of the plot or world develop since that in itself gives something away.
Spoilers spanning books 3 and beginning of 4
I had to go back to check how this rivalry was set up as it was a bit gradual. It was the Book 3 Prologue where we meet Beniko’s rival よどみ of the たたりめ堂. Although at that point I’m pretty sure we don’t know they’re rivals. Maybe, I’d need to read it again but I think we find that out in the book 3 epilogue. Although there is at least a hint as we find out she makes her sweets from the ill fortune bugs that come out of the vials Beniko puts the payment coins in. The good ones become the golden cats and the bad ones become these black oozy creatures that run away.
Book 3 epilogue
It’s the book 3 epilogue where Yodomi confronts Beniko and we get the start of the rivalry. It wasn’t too specific, but it shows they have different visions of why they do what they do and how much they respectively care about their customers.
Yodomi confronted Beniko since Beniko’s sweet in Ch 1 counteracted the effect of Yodomi’s in the prologue
book 4 prologue +ch1
Haha, me too! I think before I thought she was mostly benevolent. I still think she has overall good intentions, but this showed more of a neutral stance if that makes sense.
That’s right, right from Ch 1 here we see how the characters get caught in it.
Yes! Very much explicitly darker, the key is in her word choice in payment and how that payment plays out.
Generally
Her payment is the ill will that results from her sweets. And she (literally) extracts her payment at the time it all goes wrong.
Specifically in ch 1
Specifics: When the boy freaks out when the teacher asks him to explain his correct answers on the test, Yodomi reappears and there is a black fluid she extracts from him. I wasn’t sure is that was the sweet or the ill will, maybe both
