I thought the same thing, Porgy. This is a ghost story. When Shasta comes back for the couch scene, she could be a dark specter. A shadow. Also, she is a fantasy, what Doc thinks she was to Mickey. And we can also think he's hallucinating because, well, it strangely looks like a variation of the first scene. Of course, I'm with you, I hate the "THAT CHARACTER DOESN'T EXIST" theories, and in the book the fogginess of Shasta's presence is different. But Doc is haunted by Shasta. Shasta and the hippie's dream are fading; I love the last shot of the movie. In the book, Shasta isn't with Doc. But I'm not sure she's with him at the end of the movie either.
That's why Coy is important. Coy is, for Doc, a way to have a victory against death/corruption/what's going away. Coy is resurrected.
About what it says about America, it is in the movie. Strolling talks about "vertical integration", does she? Or maybe it isn't the exact term. Vertical something, anyway. America itself wins money with drugs with the help of the federals then wins money again fixing their teeth before transforming them into anti-communists with the help of nazis. Or something like that with different connections. Doc says: « Much, much more, what you would call, vast. », it isn't clear to give you an idea of the vastness.
With the boat, you have the idea of a sudden transformation by evil, hidden forces. What happened? How did it go away? (Sashta in the first scene/couch scene, the communist actor/anti-communist actor, Wolfmann totally broken after his "dream")
The nostalgia of America is harsh; at the end, Sortilège says there is no way to go back, basically, that we have to live in this world. And this nostalgia works, to me, because of the ex-old lady. Something I connect to. This isn't only about 60/70 the same way TWBB isn't only about capitalism. It's the feeling of something that goes away. An era or a lost love.
Look at Bigfoot. He hates hippies but he's a victim of America. Adrian Prussia is loved by the feds, and he killed his partner. Doc lost Shasta. And Don and Bigfoot become, in a way, a couple of broken couples -- if what I say makes some sense. Not a real couple (most of the time they're on the phone), but something strange. I didn't laugh when Bigfoot ate the weed, I felt like Doc. Where does Bigfoot belong? No-fucking where.