You know, Ironweed reminded me so much of Beckett's Endgame, with its pathos and fatalism. And a little of Death of a Salesman, with the futility of work and family. You're right, it was painful to read. Which is often the case with "literature." I guess there's no such thing as a happy ending any more.
So here are some quotations I found thought-provoking:
"What brought her to this uncertainty is the accumulation of her sins, and if you must call them sins, then there is certainly quite an accumulation. But Helen prefers to call them decisions, which is why she has no compulsion to confess them"
"If you love something well enough [...] you will die for it; for when we love with all our might, our silly little selves are already dead and we have no more fear of dying"
"Fornication was standard survival currency everywhere, was it not?" (Which you already referenced)
"Francis said to his unavailed-for self, and he smelled his own uncanceled stink again"
You questioned the relationship between spirituality and the poor. I only saw the relationship between life and death. I thought everything circled around to that idea, so much so, as a reader and/or character in the book, you are never sure who is alive and who is dead. The free exchange of sex in the book seemed to reinforce the idea that life is futile and meaningless. Everything--dialogue, actions--seem mechanized and robotic in a sense to me. That's where I sensed the whole Endgame vibe--everyone is already walking around dead, therefore speaking of sin is irrelevant because you must be living for sin to "count." There were two scenes that resonated with me--the Mrs. Robinson scene where Francis is seduce by his older neighbor woman. When she tells the story of the caged woman pulling the rabbit apart with her teeth. Grotesque. Also, the scene where he eats turkey with his family. Then willingly walks away from them again to embrace his vagabond lifestyle full of STDs and alcoholism on the streets. There is no hope in the book, only death and carnage. Why did you pick this book?? Kidding, it was, as you said, well-written.

Here's my last comment on the conch shell: just mentioning Freud does not mean that I am speaking of the biology of castration and/or the phallus. The phallus can be a symbol for power in general. So yes, the emasculation of civilization, or of any structure in general is exactly to what I was referring. Also, I don't believe half of the stuff I write about Freud or deconstruction, I just think it's fun to talk that way.