Good questions and insights Andrea!
Not sure if you know Plath's story - perhaps your questions point out the flaws in her thinking - she saw things in a rather limited way, which is why she took her life at an early age. Not sure if that makes her an existentialist in your book, but certainly someone who did not see everything quite clearly. But I still like the image - that making a choice is important to progressing in life.
Wow, is your Mom a blessed lady to have such a dedicated daughter as you! I think many of our choices are like that - we choose without really understanding or being aware of our choice. It is clear that God has gifted you with a lot of PEACE in your choice, as you have exhibited that in the forums!
I wasn't familiar with her story. I did look now on Wikipedia and skimmed some of her info. I don't know if she was an existentialist or not - it just seemed the focus was on life being choices, that it doesn't matter what the choices are, just that you make them - that sounded existentialist to me. And from reading about her - it could've been true, a life devoid of God. I don't know.
I do believe looking up and saying God help me out of this hole I've dug - could be very helpful to those with depression.
I think she may have had trouble no matter her choices.
I believe what I "do" follows from what I "believe". Such that, if she didn't know what she believed and only that doing was important - that may have been her problem (unless it really was a cognitive/chemical imbalance). She may not have had a basis from which to make her choices.
However, the premise that life does contain choices is a reality for everyone, independent of their belief system. We still need to decide the path to take. And a multitude of choices do exist like in the example.