(Edited because I misread part of your post)
I'm not clear why this sorority wouldn't help you with the advertising or find some alternative for you. However, I can see why they would not let you wear the letters (because you haven't crossed) or use their name in a certain way, images, etc. (because you are not members yet). From what you wrote, it overall sounds like a disorganized group, but then again, I don't know all the details. I suppose you can try to join another organization because you have not become a member of this sorority, but you might have a hard time doing so. Some sororities will not allow anyone who has completed even a portion of another sorority's membership process to pursue theirs. Further, if people know that you were pursuing one organization and now try to pursue another, you might get some heat for it. It could work out, though.