Hi Jenni, I'm really not wanting to be insensitive to her but I'm thinking that at those times she just isn't "fit for work". Not only in the sense of carrying out her job but it sounds like the "stresses" of work really aren't helping her either............so maybe some time off sick to get some real help e.g. through her doctor???
I don't know whether you'd feel able to sit down with her and try to sensitively support her in taking/suggesting that route or you think talking to a manager would be best, but she certainly needs more help than you or your colleagues are able to give her, and that's not a bad reflection on you at all. It's just from what you've said including the thin line between "screaming, crying and suicidal threats" then it sounds like she may need professional help or at least something more in the way of "experienced" support, right now. And she has admitted she needs help hasn't she?
And please don't take the way she's behaved towards you personally, it sounds like she's really hurting right now, and sometimes it's unavoidable that others are going to get caught in the "crossfire" that doesn't need to mean that you or she is to blame. The most important thing is that she gets the help she needs.

Alison