The account from the Roxie understudy that Stiefelman refused to engage with her onstage in a moment during which Roxie and the Conductor are directed to interact would also seem to be pretty serious, as it's something that would have been happening in front of an audience (not that I think Stiefelman's reported behavior in a closed rehearsal is acceptable either). That shit should've been addressed immediately after it happened by Stage Management, just as if an actor suddenly decided to change blocking during a performance.
It's highly unprofessional behavior under any circumstances, but certainly seems like Stiefelman was attempting to sabotage an understudy. I would have fired her at the end of the performance. Assuming that the actress' account is accurate, of course.
