Um I do believe that even though Harley has her eyes on Mr. J she has dated other people. For example she for a short time went out with the Creeper. So it is very well possible that something has happened between Ivy and her.
I don't think she is nieve at all. I kind of dislike that they down play her intelligence. No matter how her story is told she still has a background in psychiatry. Sometimes she has been a doctor of psychiatry and other times she has just been a student learning psychiatry. In her 38 issues she was smart enough to get kicked out of hell and told never to come back by the devil himself none the less. This proves she got brains. She has the intelligence and physical stamina to go toe to toe with batman. There is even a clip of her in B:TOS sitting in the batmobile yelling at batman going i bet you think i am just another bubble head blonde bimbo well the jokes on you, I,m not even blonde. I like a little mix of both, She can be goofy without having to play down her intelligence. If they start making her act stupid all the time people won't take her serious just like no one takes powergirl serious cause of her enormous rack. I will stick with Harley forever though. I have been with her since the beginning and i will follow her to the end.