I apologize for suggesting that this was silliness last night. PeterJ's response caused me to see that you have asked a very good question.
First and foremost, since you have formulated the question so well, you could do a phenomenology of the "Neither Dead or Alive", or of any of your topics/observations. If you are not familiar with phenomenology, then you may want to read in this book: Introduction to Phenomenology, Dermot Moran, Routledge; free at Internet Archive.
You yourself have already begun a promising phenomenology. You are the philosopher here. This may be the best way to proceed, i.e. through phenomenology yourself.
Second (my observation), the "Lebenphilosophie", why was it ultimately an ersatz philosophy?
Such philosophies as this cannot stem the tide toward reductionism. Ultimately they are not satisfying, and neither is existentialism in my opinion. One of the things you are dealing with in your post is a reduction of the fullness of being. So German philosophy, art and culture at the turn of the last century and going forward may be of interest to you in the tracing of the history of this loss. Vienna too. Look at German Expressionism. The history of positivism too. A fear sets in that Being is in danger. This shows up today in the most mundane ways, even reduced portion sizes and so on. People feel that life is shrinking.
Third, neither present or absent. How in the world did you think of this?
One form of this Being is the Jewish God. We don't see his face, maybe we see his back when he turns the corner. The idea of the Kabbalah is to fix the broken world (broken vessels) by restoring the many into the one through kindness, etc. so this is the repair and restoration of being. This unity is not exactly the same thing as you see in the Perennial Philosophy [because Ein Sof is always beyond "unity", always in some sense hidden]. So this dynamic remains ambivalent as in "neither present or absent". There is no merging into mystical unity here, good acts in this world are still important.
In my opinion we don't want a unity of the night when all cows are black anyway, this seems like anesthesia to me, but I must say that Karl Jaspers, though a Perennialist, could be helpful, his book, "Way to Wisdom", but he is not definitively helpful, imo. This falls into the category of therapy for the loss of being.
As far as these creatures in Limbo, they can write etc., but not too well about themselves. These Zombies (which are always the living-dead, not the dead come back to inform) qua Zombies have to be chronicled by those who can still in some way know. It would take a huge effort to restore the fullness of being to them though. It would require a changed society.
Art will always be heavily involved with this sadness of loss and separation and reduction. Diego Rivera's "Landscape with Cactus" may be a place to start. As I mentioned earlier, Wim Wenders (film) mastered "nowhere".