My girl was engaged... didn't stop me from swiping her away from that shmuck!
Look at it this way, if you successfully hook up w/ her, GREAT! (irregardless of the other dude competition crap, that will always be there) If not, then you know she doesn't have those feelings about you, and you can forget about her and get on with your dating life.
Sorry to say, but at this point, your intentions of wanting to be more than "friends" has most likely mucked up any "friendly" relationship you will happily have with her. From my experience, once YOU want to be more than friends and you discover she doesn't, the friendship is screwed.
Good luck! Let us know of any new development.