Yep. 生徒会長の尻穴調教日記 is pretty clearly an alternate timeline version of this series, for example.
I don't see it. There are two girls in this that can maybe charitably called prototypes for Mitsuba, but the first one is, at best, Mitsuba run through a yassification filter. She has Mitsuba's vibe in body, but every detail is made to be conventionally attractive in the same way Kasahara is and that throws everything off completely, plus she's clearly a student council president type. The second chick kind of maybe has some of the facial features but everything else is off.
Neither of them have the most important parts of Mitsuba's design: thick eyebrows, large and open downturned eyes, and a really plush and thick body. Seriously, Mitsuba is noticeably fuller in body than the other two (at least past Vol 1); her arms and legs are noticeably thicker. I'd guess Mitsuba is a non-related porn design that got used here because the artist coincidentally got a good idea for a non-H manga story while they were making her. She's clearly more
everything than the two examples, so this wasn't a case of the artist toning down their turn-ons in character design for a mass market. There is also the possibility that Akari and Saori were made as the same character in a "moral degeneration" scenario and they got split in two for the purposes of this story.