This is quite the comprehensive post. I know it's difficult to compartmentalize an entire human being but I would like to make one specific point.
Personality disorders are not mental illnesses. They are listed in the DSM-V because often they are treatable by mental illness professionals, such as behavior modification techniques or biofeedback training. However, a personality disorder has to do with how a person percieves and responds to the world.
A mental illness is a brain based issue that includes a chemical imbalance component which is why they most often respond so readily to medication. Depression, anxiety and OCD are all examples of a mental illness.
A person can suffer from both simultaneously and I have no doubt she may be exhibiting signs of both. My best advice to you is to be very careful to define your terms and try to apply the best solution to each of her difficulties.
So, for example, you can find her a psychiatrist that can not only prescribe some appropriate medication (for her depression and suicidal ideation) but can also provide the talk therapy that will help her realign her perception of herself in this world and how the world affects her (to address the possible personality disorder).
Don't walk on eggshells. She may be 18 but you are still her first and foremost advocate. Take her into your arms, show her you're on her side and that she has a partner in you to unravel these revolving and competing issues she's suffering from and take as many proactive steps you can to demonstrate this to her.
My thoughts are with you and your daughter. :-) S.