On the 23rd century’s knowledge of Nazi Germany - the show never “understood” this, but it’s quite possible that most of what the 23rd century knows about the 20th is all wrong. Lots of records could have been lost. Maybe all they had to go by about WWII is The Producers, an English translation of Mein Kamf with a bunch of pages missing, and a worn copy of The Battle of the Bulge. Their “scholarly” knowledge of the past could be as good as that of New Yorkers in the year 3000 in Futurama.
The redjack episode (Wolf in the Fold) is even more offensive - Kirk Spock and McCoy think Scotty could actually be the killer, because “it was a woman that caused the accident that nearly killed him.” THAT is cringe-worthy, and incredibly offensive, even in the 60s. A little bump on the noggin, and heck, any man could turn into a knife murderer.
I think the only reason they made the space Nazi episode was because they had all those uniforms and props on hand from Hogan’s Heroes. It was cheap. Plus it was close enough to the war that Nazis hadn’t evolved onto the mythical supervillains they have become.*
As for the Prime Directive, I support it for the example given, for primitive cultures. Maybe I’m still stuck in primitive thinking, but saving some people and not others IS interference. Who decides who lives and who dies? Do you want the Feds to be like Kodos the Executioner? The Vians? You save that nice Edith Keeler, because her views on pacifism match yours, and look what happens. You save that nine-year old Dolf from drowning, because you think he has talent as a painter, and look what happens. Letting cultures find their own way is the only way. Who could you trust to have enough wisdom to make those decisions? Not humans, not Q, not the Thasians, not the Organians. The Doud? Best to let nature run its course.
The show’s idea of keeping it for space faring cultures is, of course, absurd.
*I wish I could remember the source of this quote - some vampire movie, one character asks why, if vampires are real, why no one remembers them, believes they exist, if they were around only a hundred years or so ago. The answer, to the effect of “hell, in a hundred years, people won’t believe Nazis were real!”