Of course she's depressed. She can't graduate high school and she can't go to college because of it. She can't get much of a job without an education, and she doesn't have to get a job because her parents are funding everything, both essential and non-essential.
You are using her fears and her threats as justification for not doing the hard work of parenting. You let her get away with it because it's easier for you. And by "you" I mean both you and your husband. He might talk a tougher game but he's not following through either. You are letting your fear of her following through on her suicide threat be your rationale for letting her party with her friends.
She doesn't want to go to therapy because it's WORK. You have to look at your shortcomings there and be honest, and she doesn't want to. Unfortunately, you and your husband are the same way. It's harder to do that, and it's easier to just keep going as you all have been.
Is it fair to assume that you pay her bills, and her wages from 25 hours a week go to pay for her fun?
You need some help getting a backbone and recognizing that you and your husband are handicapping her. Your babying of her is actually contributing to her depression, not solving it. So get some family counseling, and set a firm timetable. That can include a 30 day notice, after which you cancel her cell phone (which she doesn't need except for friends), pack up her belongings and put them in a storage unit (you put it in her name and maybe pay the first 1-2 months and let her know that they will auction off her stuff if she defaults on further bills), you transfer the car insurance bill into her name, you change the locks and take her key, and you don't deal with any screaming or lamenting. But you only do this if you plan to follow through, otherwise it will set you back and you'll apologize by paying even more bills.
She is perfectly capable of living with these terrific friends or getting a furnished room, she is perfectly capable of getting a second job, and she is perfectly capable of getting help for her depression. Anesthetizing her pain with parties (and alcohol? pot?) isn't helping herself. If she says she can't move in with friends and impose on their parents, then she knows it's wrong!
I would not pay one dime for college - she has not shown that she will attend, and doing on line courses has even fewer checks and balances than attending classes in person. She has not shown that she will do anything at all that she doesn't want to. You can certainly take a page from her book and say you aren't going to do anything you don't want to either, including crippling her for life.
If everyone is an adult here, then everyone can undertake adult responsibilities as well as adult pleasures and privileges. The first step is for you and your husband to take that position and believe it. Until you do, this situation will continue.