I don't agree. Her acting is definitely lacking here.
I will continue watching but I can definitely judge an actor's acting based on the 2 hours that I watched. I can't exactly put my finger on why I think her acting isn't suitable for this role, maybe because she's not charismatic enough to make us believe that both MLs would just fall head over heels for her really fast? And I don't really agree with you saying her character is supposed to be stiff like this. Her character is definitely struggling with her decisions too so if she looks stiff, it's because of her acting.
No, it's not part of the main plot. This drama is mostly about bromance not romance🤭