Izz although I don't doubt Seju has done manipulation to Sumin, I think you are reading way too much into turning everything single thing going wrong with Sumin into Seju's fault.
You are wrong that she made it difficult for Sumin not to sleep with her. Sumin wanted to sleep with her, and she wasn't physically pushed whatsoever. Stop blaming this to Seju. Seju didnt rape her or anything. Sumin willingly participated as well. They are both adults and fully aware of their actions.
In fact, I think what Sumin did was worse than Seju for two main reasons. She was the one in a relationship and the cheater, and also she was been cheated before and knew how awful that was. She kept on beating Seju for cheating on her for ten years, yet she still chose to cheat herself too. Hypocritical much.
And we still don't have any proof that Seju planned for Sungji to catch them together. Even if she wanted to, she couldn't be sure that Sumin would agree to do it with her beforehand. And also she didn't know Sumin wanted to resign when she said they need to talk, so to me it seemed more like a whim of the moment to ask that from Sumin. If she really had a great plan to destroy their relationship, wouldn't she do more than just that after the cheating situation? She should ay least fire Sungji from the company, or force Sumin to move out of her house. Yet, she said she would give the house free for Sumin and Sungji to live together. If she was such a manipulator, she would def not make it so easy for Sumin to live happily with her gf.