Yeah, its not Klein as "The American" in the Russian prison. They made it pretty clear he was on trial for corruptions and possible cover up of the deaths of Bob and Barb. Would kind of be a shitstorm if he just disappeared.
I've read some people claiming that the post credits Russia scene could potentially be out of chronological order and takes place more around the time when Eleven first opened the Hawkins gate. That to me doesn't make sense, because at that point Eleven was already tasked with spying on the Russians in the Upside Down and Dr. Brenner would have found out the information that the Russians held an American prisoner who knew about the gates.
It is very clearly Hopper, who likely jumped into the gate to avoid the explosion since he was aware what the upside down was like from his experience there before. You also have to keep in mind that Dr. Owens (Paul Reiser) appearance at the end is no mere coincidence. It is entirely possible that there were some evacuation procedures should they ever need to be pulled from a gate in an emergency and Hopper was clearly in touch with the military should he need backup for the very situation that occurred in Hawkins during S3.
Robin was best new addition to the cast, and Steve does continue to be a great character. I'm sure the writers have such a ball writing lines for him and Robin's addition made both of them better.
Chicago as the new jump off point for the Byers I don't believe is a coincidence. Eight will likely return for S4 now, and it gives them many options for opening up the story to close the series out in S4, or lead into a movie. Would be interesting to see if Eight's powers have decreased or disappeared now that the gate is closed, and how the two of them can regain it.
I spent a big part of this season, and some of the time off between S2 and S3, wondering what role the Demogorgons had. They were so important in S1 and then ignored in S2, so it was great to see their relevance return and hopefully explain their connection to the Mind Flayer and the UD in general.
Billy was okay as a villain, and in that I mean they did a really good job with the hivemind aspect that I always just saw him as part of the collective that just happened to have an attachment to the opposing force in his sister. I didn't care about his death, but him dying didn't feel cheap at all. It worked.