] Mrs. E takes over as principal at the ball, when Emma expected to do so. What gives?

Being the obnoxous self-centered person that she is, Mrs. Elton just assumes that the ball is being given in her honor.

Also, Mrs. Elton has a slightly higher social status as a married woman. Remember in P&P, Lydia tells Jane that as a married woman she now goes ahead of Jane as they go in to dinner.