Katharine would have been a great Joan, but I can see why Ingrid was chosen many years later. Ingrid is great at expressing strength AND vulnerability. Katharine is just such a powerhouse of badassary, that Joan's low points in the story would have been tough for Katharine to pull off convincingly. We'll never know, but Ingrid was fantastic in her film.
A screen test with no dialogue? Were they going to make a silent film in 1934? If those so called geniuses at RKO had agreed to hire George Bernard Shaw to write the screen play for his own work,as he did for "Pygmalion" in 1938,and "Caesar and Cleopatra" in 1945,"St. Joan" would have been made in 1934.In both cases,Shaw's screen play improved the original play.Katharine Hepburn would have been an excellent choice for the role of Joan,but I would have liked to see Margaret Sullivan or Loretta Young tested also for the role.It would have been interesting to see.
I see why they, character-wise, might have thought her apt for the part. But she would have been totally miscast had she been given the roll. Her roll is feminist patrician, not fire-brand rebel.