I'm working on a biography of Sergeant and can perhaps partially answer your questions.
Cather and Sergeant were very friendly from 1910 until Sergeant was sent to France as a correspondent for the New Republic in 1917. When she returned in 1920, they continued on friendly terms, but the friendship had cooled noticeably, and they didn't see each other nearly as often. (Sergeant was also away from NYC for long periods of time in the 1920s and '30s.) Those wonderful letters Cather sent ESS from Winslow are very much the best of the lot; it was not a lifetime correspondence.
I read the ESS/Lowell correspondence a couple of years ago and would have to review my notes to be sure, but my memory is that the subject of Cather doesn't come up often between them. I do know that there is a Cather letter, I believe to Mabel Dodge Luhan, in which she writes that she has just learned that Lowell died, and she will have to apologize to ESS because she recently said some snide things about Lowell in ESS's presence--at a dinner party, I believe. The reference, while brief, is very sympathetic.
From: 5 Bank Street: The Listserv for Willa Cather Scholars [mailto:[log in to unmask]] On Behalf Of Becky Roorda
Sent: Sunday, June 09, 2013 12:17 PM
To: [log in to unmask]
Subject: Re: [5BANKSTREET] Letters!
OK, so I'm replying to my own post--but I simply had to say how much I'm enjoying these letters. I'm savoring them, so I'm only up to early 1917, currently reading a couple of letters Cather wrote to her mother. These letters strike me as dutiful and sweet, newsy letters about things she thought would interest her mother. Cather was a good if unconventional daughter, and I don't think she ever gave herself a break about that. As a daughter of a very "eccentric" 88-year-old mother, I have some sympathy.
My favorite review of this volume, so far, is by Jennifer Howard at The Chronicle Review, someone who actually did her homework and interviewed people who had knowledge of the project.
What have I found interesting so far? Well, some of my favorite letters have been the ones that Cather wrote to Elizabeth ("Elsie") Singer Sergeant (c. 1915 or so, before and after). I was struck by how Cather frequently mentioned Isabelle McClung in these letters, yet she NEVER mentions Edith Lewis. Just an observation--interesting, I thought. Does anyone have any insight into that?
ESS was a friend of Amy Lowell, and Cather was an "enemy" of Amy Lowell. I once thought it would be interesting to find the letters between ESS and Lowell and see if Cather is mentioned in any of them--but it's a project I never got around to. I also wish that someone might do a volume of letters between ESS and Cather, putting them in context, etc. Is there anyone out there working on ESS? She is the older sister of Katharine Sergeant Angell White, fiction editor for the New Yorker from about 1925 to 1960, so that fact adds an interesting dimension. Anyone interested in magazine publishing in the 1920s-1930s could have some fun with this.