There are some very in-depth questions there, a bit much to answer all in the space I am provided. Therefore, I will deal with Q2 - the development and revelation of animosity of Mrs. Slade towards Mrs. Ansley.
Mrs. Slade is a jealous personality. She clings hard to what she has and envies what other's have. Throughout the evening of the story, she demonstrates envy over Mrs. Ansley daughter. Mrs. Slade's own daughter is quiet, serious, and unexceptional. Mrs. Ansley's daughter, Barbara, is intelligent and out-going.
During their conversation, Mrs. Slade not only becomes more upset by the "wonder" of Mrs. Ansley's daughter, but also by Mrs. Ansley's belief that Mrs. Slade is unhappy with her life. Mrs. Slade is, of course, but she doesn't want others to know that.
Because she is feeling inferior now, Mrs. Slade reveals the secret about the letter from all those years ago. She lets Mrs. Ansley know that she did not trust her. Mrs. Slade always felt that Mrs. Ansley was interested in her fiancee, and she wanted to not only trap Mrs. Ansley, but even cause her to get sick so she would be out of the way for awhile. Then, Mrs. Slade could keep Delphin to herself and make sure she "wins".