I was initially cold on the idea of the BCR being the dominant rebel group in the JPS (not to sound like a 'gamergate' type, it seemed more like it was put in to be 'woke' than plausible), but the idea has grown on me a bit. It makes sense that there would be considerable white-flight from the JPS to the Reich and considerable movement of black refugees westward. The 1940 population of California was 6.9 M. The black population of the US in 1940 was almost 13 M. It wouldn't take all that much of the black population fleeing west to give them local majorities in much of the state, particularly if many white Americans leave.
If the Japanese were smart about it, they could have had a very pliable subject population in the refugee blacks--a group of people already hostile to the white population, and of near-guaranteed loyalty when the alternative is the Nazis. But, apparently, they weren't.
The obviously-Soviet weapons and the fact that China is supposed to be a major threat to Japan in the 1960s raises too many questions about China. Honestly, the most plausible explanation is that Mao's China has its own portal through which its funneling lots of weapons and soldiers. Realistically, with the Nazis in complete control of Europe and America occupied, China should have disintegrated by 1950 under the strain of a decade of nonstop war and such things as the Henan Famine.
The immediate aftermath of Smith's death is likely a long power struggle and consolidation by a junta of generals in the reconstituted US Army. Probably Whitcroft in charge. It's to be noted, though, that between them, Smith and Goertzmann have culled the majority of recognizable charismatic would-be Fuhrers--Heydrich, Hoover, Rockwell, Himmler, Heusmann, all gone. The military junta is going to be a bit weak-kneed without someone like Smith to unite them. Whitcroft is the closest thing to such a figure--the conquest of the Neutral Zone makes him a war hero and someone around whom 'Americanist' Nazis can rally. If he avoids assassination, he can probably consolidate his position enough to complete his goal in a few years...
Assuming the Japanese don't extend their nuclear umbrella over the JPS, which they might. The BCR and their allies might realize just how unstable their position is, and sheepishly ask for that back in exchange for trade deals very advantageous to Japan.
Which brings me to my complaints about the season: As I said, the BCR grew on me, but they should have been built up longer. They should have done more than one good strike on an oil pipeline. Ideally, they should have been built up in previous seasons, but if this was a late idea, they should have spent a whole 10 episodes building them up as a threat to the Japanese. Maybe also explain the changed demographics of California as I suggest. The season should have been stretched another few episodes, maybe to 15, to allow for this and not make the Japanese Empire look like it has a glass jaw.
And that ending with the people coming out of the portal--that's just baffling. My own pet theory is that those are people who died in the Man in the High Castle timeline (in German camps and Japanese killing fields), but lived in other universes, but that's really not all that satisfying--why should they just be allowed to cross over? That could have used some build-up, or just been left on the cutting-room floor. A better ending, IMO, would have just been to follow through with toppling the Nazi American Reich and establishing some kind of real free government, even if that does clash with the BCR's stated goal of not being the old America--which is a good thing in the show, so admittedly it's a bit hard to square this circle.
But back to the good stuff: Sewell's performance as John Smith is still the show's best element. He brings such a pathos to the role that one almost forgets just how monstrous he really is. Almost. It didn't even occur to me until about a minute before Helen tells Smith to stop (the genocide) and he answer's "I don't know how" to stop and remember that, as Reichsfuhrer, he doesn't actually have to go through with building death camps. He's in charge. He's eliminated his opposition. He can scale it back--slave labor, deportation--he doesn't have to fire up gas chambers. Is he a committed Nazi? The show seems to lean toward no, that he's justified everything he's done with 'for my family.' But the fact that he doesn't stop calls that into question. Is he trying to keep some constituents in line--but who? Rockwell is dead and the Germans have given him autonomy. He doesn't answer to anyone. Everyone who knew about his misgivings except Goertzmann is dead, so there's no one he has to prove his ideology to. Does it just never actually occur to him? That's plausible--he is so engaged in his Nebenwelt project that he might actually not be giving the invasion much attention.
I wish there had been some meeting between Smith and Alt-Smith, but the show ruled that out last season.
His daughters were also played well, Amy in particular having the creepy vibe we should associate with Nazi children.
Childen is still the collaborator extraordinaire, just the right mix of sad, pathetic, and sympathetic.
Juliana...meh, she's still not all that interesting compared to the others. More interesting things happen around her than anything she does is interesting.
Kido's arc with his son was a satisfying end.
Thoughts on the aftermath (ignoring the portal-people and disregarding my hypothesis of Mao's China having its own portal):
By the mid-1960s, I expect Whitcroft to be solidly in command of a Nazi American Reich that includes the former Neutral Zone. A lot of the Jahr Null policies will be rolled back and a much more American-flavored Nazism will emerge. But they will be unable to push through into the Western States because those will have reentered the Greater East Asia Co-Prosperity Sphere for its nuclear umbrella. Japan will have crushed the Chinese with reinforcements from across the ocean and have secured its position in Asia. Both them and the Nazis will have tested a hydrogen bomb, so a new cold war will settle into place. Nazi America will be a third power, kind of akin to OTL China or France--its own nuclear arsenal and geopolitical agenda (like China), but aligned with the Nazis (France was never really going to join the Warsaw Pact). The Germans are said to still be fighting military campaigns in the Urals, so some kind of Soviet government still exists--the Japanese will probably be supplying and arming that one after China is crushed, to keep the Nazis busy.
Sooner or later, the Nazis' habit of massacring their leaders in bloody coups is going to send them into an actual civil war, which will probably go nuclear and escalate to a global thermonuclear war. I predict that that will happen by 1980.