there is much more backstory in the book. actually, it's hardly backstory. they don't start as hopeless addicts; they break their rule and start progressively abusing their selling supply. the mother's stay in the psych ward is fleshed out as well -- much uglier and not everyone involved with her treatment is onboard with the ECT, but where is her drug-knowledgeable kin to sign off on one decision or another? that said, the book and the movie are pieces independent of one another.
you need backstory to accept that a group of friends have become significantly addicted to heroin? and that the resulting world is cold? and yeah, that isn't the theme. it's the vehicle. theme is that the insatiable human-need to consume leads to self-destruction and creates a "frigid, barren place where everyone and everything is going to fuck." the writer would tell you this is life; there is one way out. that's fine to disagree, but realize you are focusing heavy on vehicle over theme.
they are not looking for a bundle. the ring they were operating in is destroyed. the source decides to sell directly, but finds it cannot work with the area in the resulting structureless state. the friends can't spend their last bit of money on folds, stamps, bags, whatever. they need to score raw in weight or they will forever be in need. or that's how they feel. yeah, she can fuck a dude for some h. they can walk down the street and cop cut-up, overpriced street dope -- what do you think he's shooting on the way down to florida? that's not what they need. or that's not what he cares about. there is a schism on where the dope money needs to go. she is further lost than he.
na, they don't need to treat you for shit before they send you to jail. and i don't think the audience is to assume they were arrested for an abscess. it's reasonable to assume there is dope in the car. we know there are used needles. shit, i know someone who got arrested for (alleged) dope rubbed into their pants. and that wasn't a black guy down south.
along the lines of what you said about the american dream, the movie is focusing on certain western values and the society and individual those values create.
the movie doesn't give a fuck about HR. so that's a weird lens to watch it through. might play a part in developing a negative opinion about the movie.
edit:
that ER to incarceration scenario is dated. i have a hard time seeing it go down quite that rough. when i said "na, they don't treat you for shit before they send you to jail," i meant, "you'd be surprised what and how long something can go untreated for an arrested or incarcerated individual."