The more I learn about WWII, the more I feel like any AU of that kind grossly overestimates their competence.

Absolutely. Leaving aside all moral aspects: there was a staggering amount of corruption going on, and such a lot of internal feuds in the party hierarchy, plus all the cult aspects were so Hitler centric, that I think it would have collapsed within a few years even in these scenarios where the US doesn't intervene and the Hitler-Stalin-Pact is maintained. Which is not to say they wouldn't have murdered a lot of more people before doing so! Also, if you look at the fascist dictators from the 30s who actually survived and died in their beds decades later, i.e. Franco and Salazar (the Portuguese guy): it's not that they were better human beings than Hitler, it's that they had a more realistic idea of how far they could go, maintain their grip on their country, and not invite alliances by everyone else against them by first attacking everyone else. Yes, if Hitler hadn't invaded Poland, no foreign power would have helped the Jews and other persecuted people within Germany. But then he would not have been Hitler. Basically, you'd have to change so many factors for an AU where the Nazis win and are able to maintain power afterwards that it becomes pointless as an AU.

Wasn't the Challenger mission, and the inclusion of Christa McAuliffe, at least in part NASA trying to get some good publicity because people had been losing interest in the space program? Which presumably wouldn't be as much of a problem for NASA in the For All Mankind universe.

Indeed!