Historically speaking, would it really have been possible for Christine to have been in the corps de ballet as well as in the conservatory studying voice? Were ballet girls even chorus girls?
In Leroux Christine was a part of the chorus, while she in the stage version has been made a ballet girl. They would have separated between those two in the 19th century, so she would have been either a singer or a dancer, not both. That is, the dancers would seldom be singing, while the chorus would do some dancing - like a chorus in a modern musical. But their main job would be to sing the chorus parts of the opera.
The role of the dancers has also been simplified in the stage version. The ballet rats alone would have been some 50 individuals, and they were children. Then they had different ranks of corps de ballet and soloists (ca number of individuals in the parenthesis): premier quadrille (19), deuxième quadrille (18), cophyrées (18), petit sujets (?), premiers sujets (28) and étoiles (I.E. the stars). The tiny corps de ballet in ALW’s Phantom serves both as ballet rats, the quadrille and the stars, and they’re only 7-8 individuals. This is a much condensed version of the corps de ballet in Palais Garnier!
But back to the point, think of it - the word chorus and the word choreography. Etymologically speaking they’re twins. Chorus literally means “band of dancers”. Choreography means “specified/written down dance”. The chorus in Greek drama were a homogenous group of speakers/singers/dancers who commented on the action through a defined set of rules. They were the emcee of sorts, but 15-50 of them acting as one. They both danced and sung, and this was also the case of the chorus in the Parisian opera some 2,500 years later.
If anyone want to dig deeper into the world of ballet dancers in Paris in the 19th century, I H-I-G-H-L-Y recommend the book “Degas and the Dance” by jill DeVonyar and Richard Kendall.