It is possible that I am biased, because I watched Expanse and ( apart from being annoyed by how it quickly it became a relationship show ) I absolutely bought into the premise including the points you listed:
* The idea that we would actively intentionally re-create the situation of MAD in another context. MAD was an accident of historical happenstance and now that we know about it, we wouldn't try to re-create it.
If there is one thing that best predicts how individual human will behave, it is how they have behaved in the past. It is sad, but that is the reality. Similarly, as a species, once we know, a certain set of actions are an option, there will be people who will aim for that set of actions. If a creation of blackholes becomes possible, you can rest assured, MAD will almost instantly will be recreated throughout the known human biome. I personally think you give humanity way too much credit than it deserves.
* The idea that a magical new engine would suddenly appear that made many of the things in the tv series possible.
Hmm, not exactly magically, but most of recent technological wonders sped up developments in other areas significantly to the point, where ( naturally with exception of fusion which is always 20 years away ) we sometimes see developments in ways that could not be imagined before ( Operation Warp speed and resulting vaccine come to mind ) save for science fiction's like Rainbow's End, where a line between development and production is.. ridiculously short. I think in the span of human existence, suddenly likely needs to be limited by definition somehow.
* The idea that Earth will for some reason simultaneously become a massive welfare state, yet still somehow have solar-system spanning power.
I don't want to be that guy, but not to search very far Soviet Russia was just such a state ( with the tech allowed to it at the time ). I am not sure how this is a contradiction. Each society governs its own priorities.
* The idea that there would be a complete backtrack in labor rights and labor safety (the ice asteroid mining scene).
I can't even.. We can just barely force corporations to maintain labor rights now with some semblance of control since we have them physically operating and we can summon the representative in court and enforce compliance. And even then, those controls are eroded via various means. Is it really that difficult to imagine 'when cats away' scenario?