You have a point, but still Remi egged her on to go down that path. She seems like she's good at doing that.
Agreed, the point being that Sacchi's still the one who chose to act so. Repeatedly.
It doesn't seem like Sacchi actually cheated.
Maybe, I don't remember how clear it was whether or not they were dating. Still, Sacchi's intention was explicitly to betray Ruki's trust, and that's exactly how Ruki felt about it.