I received a very nice email from Mirdza Kalnins' granddaughter. She sent me the following information and YouTube Video:
"She used to perform in Theatre. She was in the national Opera of Riga from 1927 till 1933, and she was going often in Paris to perfect her technique with the maestro Nikolaj Legat. Then in Rome she danced with the Teatro dell'Opera (opera theatre), as soloist and principal dancer. Here she danced Petrouchka, Coppelia, Carmen, Fedora, Principe Igor, etc...
She was part of the Vittorio De Sica's movie "Finisce sempre così", but she performed in several other movies as well. Check this YouTube video. She plays The Light in the version of Excelsior:"