Definitely makes Fuuko more sympathetic as a character.
Being toyed with and abused in this manner when she was that young makes her behavior as it pertains to Yuni that much more of a through-line. And it's not like Fuuko has allies, or any support system she could go to; so she'd be sitting in this pit for years, at this point, at the mercy of Mashiro.
Still curious as to whether Mashiro will be getting more focus, to see if there's an underlying reason she is the way she is. But perhaps it's less important--the focus of the story is Fuuko, and Yuni, and Nanase, and Yuka. Trying to delve back into the histories of previous "generations" and ancillary characters would just be a never-ending journey into the past to "justify" why they're all doing this.
Mashiro being a villain, for the sake of being a villain, seems more than appropriate, I'd argue. Not everything needs deep, long-term rationale, especially when she primarily serves as a story device to explain Fuuko's character, and the context of what she did to Fuuko is all that's really important about her character.
This backstory is important, but I do look forward to getting back to the present--leaving where we did with the other three (and, honestly, with Fuuko falling back into Mashiro's clutches as Yuni had moved to focus on Nanase) was a hell of a cliffhanger to then spend a volume on Fuuko's character.
Again, totally appropriate to have this and I'd argue imperative to the narrative overall, but I'm just so impatient to see what future trainwrecks are in store for us, haha
But, thank you for the TL work as always.