I very much agree with your observations (and clearly RKO 281 remains close to my heart).

On both an artistic and meta level, I'm sad that Fincher didn't bring Aaron Sorkin in for punch up -- whatever AS's penchant for speechifying, he's much better at distributing moments among the ensemble and there is so much ensemble to work with here. I agree with you that the relationship between the brothers Mankiewicz was the most intriguing thread in this screenplay but it wasn't allowed to center the movie.

Re: the collaborative nature of the scripting process, I think Mank does have a line about the collaborative writer's room structure at Paramount but it's done somewhat dismissively and obviously the film is more interested in Mank getting drunk and dictating to wide-eyed proteges.

One fun sidenote -- I was tweeting about the movie and the author of a book about Upton Sinclair's campaign for governor replied to me, so now I've ordered that book and am quite intrigued to learn more. I know of Sinclair as a writer but I don't recall if I'd ever even heard about that election before! Will report back.