Who was the first woman to wear pants
I know no one's ever asked me before, and I'm not sure anyone's ever cared before. Pants (trousers) have never really been a social-moral taboo, so it's doubtful their first appearance was ever noted. I don't think anyone could give you a truly definitive answer, but I'll offer you some general comments on the subject.
My field of expertise is television, not fashion, but it seems to me that World War efforts put a lot of women in pants-- think Rosie the Riveter. Theses clothes were not fashion statements, just practical. After the war, some women continued to wear
men's pants, so fashion designers started making pants tailored more to a women's form.
When commercial TV debut in 1946, the type of programs offered would have not been the place to see women in casual attire. In 1947, "Mary Kay and Johnny", the first made for TV sit-com debut. The show was about the home life of a newly wedded couple and ran for three years. Since the show was live, there are no films or tapes, but in three years, there's a good chance Mary Kay or her mother donned a pair of pants for one of their household chores.
Another staple of early TV was the western. There might have been dance hall girls and school marms in dresses, but a cowgirl wore jeans.
In 1951, when Lucy and Ethel wore pants on "I Love Lucy", no one batted an eye. Pants did become an issue in 1961 when Laura Petrie on "The Dick Van Dyke Show" was often seen in Capri pants. The brass at CBS wanted Laura to represent the average young housewife, and market research indicated these ladies were wearing dresses at home. I think the real problem was that Mary Tyler Moore just looked too damn good in slacks, because the network relented, as long as her pants did not "cup the buttocks".
Hope I was of some help. Thanks for writing, and stay tuned.