One of the grandest surviving estates of America’s Gilded Age — a sprawling Vanderbilt summer palace known as Elm Court — is preparing for its most ambitious reinvention yet. Stretching across nearly 89 acres between Lenox and Stockbridge, the legendary property was built in the late 1880s for Emily Vanderbilt Sloane and her husband William Douglas Sloane. At roughly 55,000 square feet and...