Nailpassion.com

Sound in movies has brought a new style in manicuring, for even the click of long nails, on a nervous hand, records with a definite and magnified clarity that is startling. So the long, pointed nail has given way to the short, oval one that does not extend beyond the finger tip.
https://en.wikisource.org/wiki/Page:Photoplay_(1929-07).pdf/114

Here's Lupe Velez in late 1920s

I'm not sure I agree with your hypothesis regarding the evolution of nail length/shape as loads of cinema stars had very long natual nails well into the 1970s, sometimes pointed. In regard to Lupe Velez however, it has been noted in numerous biographical accounts that she was a sexual wildcat. She was married to Johnny Weismuller the Olympic swimmer-turned-actor, famous for playing Tarzan in a series of Hollywood films. As we all know the character wore only a loincloth, and stories have it that during filming days the studio makeup artists had to camouflage the scratches on his body from the prior night's escapades with Lupe.