Mind you, I am guessing here
They probably put the comma there because the sentence is already complex (or maybe just to make it easier for kids to understand?). I’m not sure if it’s standard practice, I couldn’t find any relevant example sentences with several のs, with or without commas.
Also, from Tofugu’s article:
Comma usage in Japanese is incredibly liberal compared to English. You can stick it pretty much wherever you want a break or pause in your sentence. Just don’t abuse the power, please, it, is, irritating.
By sliding progression do you mean the system of discussing page 10 on day 1, pages 10-11 on day 2, pages 10-12 on day 3, etc.? I think you could still allow talking about previous chapters even after we’ve moved on.
I wouldn’t suggest this if the chapters were long, I agree it would be counterproductive for beginners. But since they’re all super short and self-contained little texts, I thought we could make use of the book’s own division system, preserved in either book format.
But yeah, let’s give it a few days and see how we’re getting on first 