Ooh. I hadn't spotted the Freck/Luckman split brain thing. Interesting idea.
Anyway, in answer to a couple of my own questions, just getting started:
I think the title is very relevent - the biblical phrase saying that we see ourselves through a glass darkly: that what we see through the mirror is ourselves reflected, but both obscured and we see a dark, starker, truth of ourselves.
There's the specific section where Arctor/Fred is looking through the Scanner, and he sees himself as he really is, rather than how he thinks of himself; he sees all his failings. But generally, I think Dick is saying that the use of drugs often brings out the darker, harsher truth about yourself.
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And, in respect to the druggie-ness of the book overall: my feeling is that Dick is writing what he knows, what he really knows. As he starts the book, he shows the warm, friendly glow - in places - of how taking drugs makes people feel better, feel like they're the good guys rebelling against the system, and the babbling, weird, but kind of entertaining conversations people end up having.
But, over time, as they take more and more drugs and break down more and more, they get more and more paranoid - and he does a brilliant job setting up the idea of a drug and informant system that would make you astonishingly paranoid - and where the warm and friendly space gets more and more vicious. And it feels like lots of people I've known who started with cannabis and moved on to stronger stuff; or who've taken just too much of anything. Never as far or as extreme as portrayed in the book, but it's very familiar and it's clear Dick is writing what he knows.
You can tell he still has good, enjoyable, memories of his drug-taking days, but has seen what it's done to him and his friends.