I don't think she's trying to actually marry Howard. She needs to stay engaged to Howard to stay close to him and his family (and not anger Empress Liza). It seems to me that she wants to ruin them beyond repair--reputation, finances, title, etc.. and by keeping that connection she has closer access to them. If she cuts him off, it'll be harder to do anything to them. Even though she's an unleasher and solia, she still needs to maintain her public image. Engagements are a huge factor in social standing.
But Iris said so herself, the best revenge was to marry Howard and expose them later on, but she still hesitate about that, that's why she postpone the marriage proposal. Honestly, I watched too many Morning Korean Drama where the female lead married the asshole to get revenge so I'm not really surprised there.

honestly idk why iris is thinking that they only way to get revenge by marrying that shitty asshole again,,,like girl there are so many ways like you have the f4 best bois + ur brother by your side ┑( ̄Д  ̄)┍