Sis, here is my understanding:

1. "the world and its mistress" refer to the important people in the world. "the world" refers to the important men and "mistress" their women (wives or otherwise). Basically the expression means 社交场合举足轻重的男男女女. (I got this idea from somewhere)

2. I think that Mrs. Crystie is still married to Mr. Crystie, but is cheating on him. Hubert Auerbach is her lover.

3.  I agree. "fantastic is meant to be sarcastic here.

4. Good information, thanks for sharing!