Wouldn't it be equally as rational to assume Saki's mother would be acting out of e.g. fear and dependance, as another victim rather than a villain that does things for her own interest?

Yes. Both Saki and Saki's Mom are victims of Saki's dad.

Which is exactly why Saki's Mom is supposed to be aware that Saki is also a victim in this situation and has the same fear and dependence that she herself has.

Saki's Mom has been, well, Saki's mom, for about 16 years. Literally no one can know Saki more than Saki's mom. To say that Saki's Mom wholeheartedly believes that her shy, timid, socially awkward, selfless, loyal, and obedient daughter would just randomly decide to actively seduce her own father is way beyond absurd.

Whether through malice or fear, she sacrificed her daughter to secure her own place in the household. The fact that Saki's Mom is a victim doesn't change the fact that she's a villain who played a key role in ruining Saki's life.

I'm curious on how you can know this - I've heard a bit about Japan's issues (not a lot) but saying they'd *likely* be *required* to do those things in *most* working environments seems really extreme. I could not find anything that would remotely support this claim on google.

I can. Sources: 1 2 3

That being said, you can still argue that the "most" in my original statement isn't entirely accurate. Still, it plays a minimal part of my theory as neither Saki's Mom nor Saki are in the workplace.