This is pretty good take, thanks for sharing!
On Cream's significance... I first thought he was a compass of River's sanity and, just like you, I saw him representing River's ever evolving mental state. Turns out, we, the hard-code fans of the show overthought the whole thing. I won't link to the article but you can Google for director's take if you like. It's not difficult to find.
Notice how the River/Stevie dynamic is drastically different from any other scenes in the movie. Whereas River is constantly bogged down by "every-day", his interactions and surroundings; it all goes away when he's talking to Stevie. She is the only one who can talk to him playfully, unafraid of passing judgement, but she does so in an almost charming way. "Dance, you nutter!" is probably the most powerful piece of dialogue, at least to me in that regard.
Another character that is much important IMO is Ira King. He's serving as a tether to River's madness. I'm not sure the show would've worked without this person. His remarks are often out of place, and are meant to provide slight comic relief. I find it works amazingly well.
Enjoy the ending, it's a thing all of its own. As great as the series already is what they do to untangle the murder mystery is almost cruel. The ending overtakes the entire story for a while, at least when watching for the first time. I can almost say I dislike it because of how well of a job it does taking away from the rest of the show and dimming the River/Stevie storyline. It flips the series on its head in one swift motion and reminds you that there's a murder here to be solved. I guess I shouldn't have expected anything else from a show of this caliber.