Responses
For your first question, yeah I honestly don't know why they didn't just tell her. As for your second question, it's already established that in her previous timeline she wasn't very social (didn't know how to ig) and probably had no prior exposure to romance. But in this one, she's just learning social skills and the only knowledge about love she has is from a book. Well then of course she's a bit naïve, she doesn't know any better after all.
So...why didn't they just tell her he was a piece of shit? Wouldn't that have solved everything? And why did she get so close to him so quickly, you would have thought she'd be even a little weary of strangers...