The show was written by someone who self identifies with one of the privileged children, because that was her background. That absolutely does not excuse the show’s treatment of class though. As you say, the show’s insistence that the upper crust were the focal point of resistance is deeply ahistorical. It also has a genuinely disturbing take on nazi collaborators, where genuine “Værnemagere”/business collaborators are treated either as harmless and blame free (As with Skjold Hansen) or their collaboration is treated as a minor sin in the grand scheme of things (Skjern) but we repeatedly morally condemn “Feltmadrasser”/Danish women who had sex or relationships with Germans. Indeed the show has much more ire towards resistance fighters going after actual collaborators on screen than it does the actual collaborators (And has the two righteous resistance fighters condemn such barbarism)
The show has bad politics. It tries to camouflage it by showing two unsympathetic right wingers (Ingeborg’s ex the nazi, and Maude’s brother), but the nazi is shown as solitary and unsupported by the community and kind of a loser, and the brother is fully condemned by the entire Varnæs clan and mocked by the rest of the upper crust for being pathetic and performative. And I think you’re right in that the show has done actual damage to Danish historiography in way I don’t think any other show has ever really done to any other country’s self perception.

But apart from the more important stuff, and focusing on the meaningless story analysis, I think you’re actually missing the biggest issue with Lauritz. Which isn’t that he’s portrayed as too communist to function (Which he is), but that that wasn’t all his character was until it suddenly was. He experienced full tilt flanderisation in a show that had three seasons.
In the beginning Lauritz is into politics, yes, but it is also clear that he is into politics as an extension of his deep sympathy for everyone, we see him help random strangers (Skjern, the painter, honestly everyone), stand up for those who can’t stand up for themselves (As indeed that is how he and Agnes start their relationship, Lauritz leaving work to beat the shit out of her ex-boyfriend then take her to the doctor), and generally just being helpful and nice. He is the only male character in the show shown to be actively interested in the actual lives of women around him, and while he is fully willing to die for his beliefs he also doesn’t think the sacrifice his wife and child would be making is fair to them. He’s also witty, smart, charismatic and personable.
Lauritz is also deeply flawed, he’s chauvinistic and paternalistic (His knight in shining armour routine also extends to lecturing women about the need for feminist organisation while sitting on his ass expecting them to serve him coffee and snacks), he gets obsessed by stuff (He almost missed his wedding doing communism after all), he’s naive, inconsiderate of the feelings of others (Although willing to help others when he can see an issue, he is unable to spot any issue that isn’t political or a physical thing), ungrateful, entitled (Manifesting both in small ways like his just total erosion of the difference between “Mine” and “yours” in the Varnæs family kitchens and in big ways like taking Agnes for granted), and the show fully treats his communism as a character flaw (Including showing that he will follow the moscow line wherever it goes and showing that as him being inconsistent in his beliefs). All of that made him a pretty cool character in the first two seasons imo.

In the third season he is literally taking food out of the mouth of his children to feed to champagne socialists, he leaves his children alone for hours to do communism, he refuses to get a job because communism, he’s unable to show sympathy or pride in his wife because communism, and all the while he’s shown to be on the periphery of the party and not doing anything actually important. This is a guy who, yes, was shitty and condescending to his wife for not being feminist enough or for caring about interior decoration, but he literally had a whole arc about how no principle he could ever hold is more important than her and that’s just gone.
And I think that’s in part because she, the writer, can’t pretend communism by itself is bad here. She can’t pretend the communists were in the wrong, so Lauritz is shipped off so he can’t be a resistance fighter, and during the post war years he has to be kind of a loser unconnected to the real party because then what he was doing was creating the entire basis for the cultural institutions people like.
Also if Lauritz remained a somewhat good guy that takes away from Agnes’ girlboss arc.