I think Showboat has KMK beat by several years as the first Broadway musical with a play within a play structure. However, KMK centers around one play, whereas Showboat has at least a couple going on I believe.
It’s also hard to define where the starting point for “Broadway” was. Technically, there were Broadway shows all the way back in the 1800’s, but these were mostly English plays (ie Shskespeare), melodramas, and musical reviews. The book musical as we know it today evolved gradually throught the first two or three decades of the 20th century.
I’m going to guess and say no, in the 1940’s, there was probably little to no concern among the creative team about the show being sexist or misogynistic. However, the best way to find out would be to research “the making of” the show. Check out books and academic articles about Cole Porter that quote personnel involved in the original production.
I saw the Stokes-Mazzie revival twice, but not the most recent one. However, I read some of the reviews and puff pieces for the 2019 revival and a “book doctor” came in and did some pretty significant changes to make Fred less of a chauvanistic jerk. I believe a couple lyrics were changed as well. Interestingly, I remember critics basically saying that they made Fred too neutered and that overall, by trying to be more P.C., the production lacked fire, and didn’t really succeed in making it more “feminist.” It was generally not very warmly received, whereas the Stoles revival (which also had some book changes) was universally beloved.
Hope that helps.